Enable AutoFDO.

This largely involves implementing the options debug-info-for-profiling
and profile-sample-use and forwarding them on to LLVM.

AutoFDO can be used on x86-64 Linux like this:
rustc -O -Cdebug-info-for-profiling main.rs -o main
perf record -b ./main
create_llvm_prof --binary=main --out=code.prof
rustc -O -Cprofile-sample-use=code.prof main.rs -o main2

Now `main2` will have feedback directed optimization applied to it.

The create_llvm_prof tool can be obtained from this github repository:
https://github.com/google/autofdo

Fixes #64892.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -286,6 +286,9 @@ impl<'a> GccLinker<'a> {
             config::OptLevel::Aggressive => "O3",
         };
 
+        if let Some(path) = &self.sess.opts.debugging_opts.profile_sample_use {
+            self.linker_arg(&format!("-plugin-opt=sample-profile={}", path.display()));
+        };
         self.linker_arg(&format!("-plugin-opt={}", opt_level));
         self.linker_arg(&format!("-plugin-opt=mcpu={}", self.target_cpu));
     }