compiler: fix few needless_pass_by_ref_mut clippy lints

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\asm.rs:306:28
    |
306 | fn err_duplicate_option(p: &mut Parser<'_>, symbol: Symbol, span: Span) {
    |                            ^^^^^^^^^^^^^^^ help: consider changing to: `&Parser<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\asm.rs:318:8
    |
318 |     p: &mut Parser<'a>,
    |        ^^^^^^^^^^^^^^^ help: consider changing to: `&Parser<'a>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\assert.rs:114:25
    |
114 | fn parse_assert<'a>(cx: &mut ExtCtxt<'a>, sp: Span, stream: TokenStream) -> PResult<'a, Assert> {
    |                         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'a>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\asm.rs:32:10
   |
32 |     ecx: &mut ExtCtxt<'a>,
   |          ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'a>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\test.rs:99:9
   |
99 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\source_util.rs:237:9
    |
237 |     cx: &mut ExtCtxt<'_>,
    |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\format.rs:809:10
    |
809 |     ecx: &mut ExtCtxt<'_>,
    |          ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\format.rs:737:10
    |
737 |     ecx: &mut ExtCtxt<'a>,
    |          ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'a>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\format.rs:68:24
   |
68 | fn parse_args<'a>(ecx: &mut ExtCtxt<'a>, sp: Span, tts: TokenStream) -> PResult<'a, MacroInput> {
   |                        ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'a>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\format.rs:607:10
    |
607 |     ecx: &mut ExtCtxt<'_>,
    |          ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\edition_panic.rs:43:9
   |
43 |     cx: &'cx mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\concat_bytes.rs:11:9
   |
11 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\cfg.rs:38:22
   |
38 | fn parse_cfg<'a>(cx: &mut ExtCtxt<'a>, span: Span, tts: TokenStream) -> PResult<'a, ast::MetaItem> {
   |                      ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'a>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\cfg_accessible.rs:13:28
   |
13 | fn validate_input<'a>(ecx: &mut ExtCtxt<'_>, mi: &'a ast::MetaItem) -> Option<&'a ast::Path> {
   |                            ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_builtin_macros/src/source_util.rs

@@ -234,7 +234,7 @@ pub fn expand_include_bytes(
 }
 
 fn load_binary_file(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     original_path: &Path,
     macro_span: Span,
     path_span: Span,