Replaced use of #[rustc_error] with // skip-codegen.

This test specifically notes that it does not want to invoke the
linker, due to the way it (IMO weakly) exercises the `#[link=...]`
attribute.

In any case, removing the the `#[rustc_error]` here uncovered an
"invalid windows subsystem" error that was previously not included in
the transcript of diagnostic output. So that's a step forward, (right?).

src/test/ui/feature-gate/issue-43106-gating-of-builtin-attrs.rs

@@ -40,7 +40,7 @@
 // inputs are handled by each, and (2.) to ease searching for related
 // occurrences in the source text.
 
-#![feature(rustc_attrs)] // For `rustc_error`; see note below.
+// skip-codegen
 #![warn(unused_attributes, unknown_lints)]
 #![allow(dead_code)]
 #![allow(stable_features)]
@@ -584,7 +584,7 @@ mod export_name {
     #[export_name = "2200"] impl S { }
 }
 
-// Note that this test ends with a `#[rustc_error] fn main()`, so it
+// Note that this test has a `skip-codegen`, so it
 // will never invoke the linker. These are here nonetheless to point
 // out that we allow them at non-crate-level (though I do not know
 // whether they have the same effect here as at crate-level).
@@ -835,12 +835,12 @@ mod type_length_limit {
     //~| WARN crate-level attribute should be an inner attribute
 }
 
-// Since we expect for the mix of attributes used here to compile
-// successfully, and we are just testing for the expected warnings of
-// various (mis)uses of attributes, we use the `rustc_error` attribute
-// on the `fn main()`.
 
-#[rustc_error]
-fn main() { //~ ERROR compilation successful
+
+
+
+
+
+fn main() {
     println!("Hello World");
 }

src/test/ui/feature-gate/issue-43106-gating-of-builtin-attrs.stderr

@@ -1303,13 +1303,7 @@ warning: unused attribute
 LL | #![proc_macro_derive          = "2500"] //~ WARN unused attribute
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: compilation successful
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:844:1
-   |
-LL | / fn main() { //~ ERROR compilation successful
-LL | |     println!("Hello World");
-LL | | }
-   | |_^
+error: invalid windows subsystem `1000`, only `windows` and `console` are allowed
 
 error: aborting due to previous error