Rollup merge of rust-lang#155389 - Zalathar:flag-macros, r=mati865

Simplify macros for target-modifier and mitigation flags

- Rebased and revised version of rust-lang#154501.
---

The macros used for handling command-line flags that are “target modifiers” or “mitigations” are quite complicated, and can be significantly simplified by tweaking their syntax and by making use of `${ignore(..)}` metavars.

It's possible that more code could be moved out of macros (e.g. declaring some of the enums by hand), but that can be investigated in a potential follow-up.

There should be no change to compiler behaviour.

Author: JonathanBrouwer

compiler/rustc_session/src/config.rs

@@ -2641,7 +2641,7 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
     if unstable_opts.retpoline_external_thunk {
         unstable_opts.retpoline = true;
         collected_options.target_modifiers.insert(
-            OptionsTargetModifiers::UnstableOptions(UnstableOptionsTargetModifiers::retpoline),
+            OptionsTargetModifiers::UnstableOptions(UnstableOptionsTargetModifiers::Retpoline),
             "true".to_string(),
         );
     }

compiler/rustc_session/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(default_field_values)]
 #![feature(iter_intersperse)]
 #![feature(macro_derive)]
+#![feature(macro_metavar_expr)]
 #![feature(rustc_attrs)]
 // To generate CodegenOptionsTargetModifiers and UnstableOptionsTargetModifiers enums
 // with macro_rules, it is necessary to use recursive mechanic ("Incremental TT Munchers").