Rollup merge of #122843 - WaffleLapkin:semicolon-vs-the-never, r=compiler-errors

Add a never type option to make diverging blocks `()`

More experiments for ~~the blood god~~ T-lang!

Usage example:
```rust
 #![allow(internal_features)]
 #![feature(never_type, rustc_attrs)]
 #![rustc_never_type_options(diverging_block_default = "unit")]

fn main() {
    let _: u8 = { //~ error: expected `u8`, found `()`
        return;
    };
}
```

r? compiler-errors

I'm not sure how I feel about parsing the attribute every time we create `FnCtxt`. There must be a better way to do this, right?

compiler/rustc_span/src/symbol.rs

@@ -691,6 +691,7 @@ symbols! {
         dispatch_from_dyn,
         div,
         div_assign,
+        diverging_block_default,
         do_not_recommend,
         doc,
         doc_alias,
@@ -816,9 +817,7 @@ symbols! {
         fadd_algebraic,
         fadd_fast,
         fake_variadic,
-        fallback_to_never,
-        fallback_to_niko,
-        fallback_to_unit,
+        fallback,
         fdiv_algebraic,
         fdiv_fast,
         feature,
@@ -1230,6 +1229,7 @@ symbols! {
         new_v1,
         new_v1_formatted,
         next,
+        niko,
         nll,
         no,
         no_builtins,
@@ -1238,7 +1238,6 @@ symbols! {
         no_crate_inject,
         no_debug,
         no_default_passes,
-        no_fallback,
         no_implicit_prelude,
         no_inline,
         no_link,
@@ -1556,7 +1555,7 @@ symbols! {
         rustc_mir,
         rustc_must_implement_one_of,
         rustc_never_returns_null_ptr,
-        rustc_never_type_mode,
+        rustc_never_type_options,
         rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,