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_llvm/src/attributes.rs

@@ -263,6 +263,10 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
         attributes::emit_uwtable(llfn, true);
     }
 
+    if cx.sess().opts.debugging_opts.profile_sample_use.is_some() {
+        llvm::AddFunctionAttrString(llfn, Function, cstr!("use-sample-profile"));
+    }
+
     // FIXME: none of these three functions interact with source level attributes.
     set_frame_pointer_type(cx, llfn);
     set_instrument_function(cx, llfn);