Use llvm.compiler.used insetad of llvm.used

The #[used] attribute explicitly only requires symbols to be
retained in object files, but allows the linker to drop them
if dead. This corresponds to llvm.compiler.used semantics.

The motivation to change this *now* is that https://reviews.llvm.org/D97448
starts emitting #[used] symbols into unique sections with
SHF_GNU_RETAIN flag. This triggers a bug in some version of gold,
resulting in the ARGV_INIT_ARRAY symbol part of the .init_array
section to be incorrectly placed.

compiler/rustc_codegen_llvm/src/base.rs

@@ -157,12 +157,12 @@ pub fn compile_codegen_unit(
             }
 
             // Finalize code coverage by injecting the coverage map. Note, the coverage map will
-            // also be added to the `llvm.used` variable, created next.
+            // also be added to the `llvm.compiler.used` variable, created next.
             if cx.sess().instrument_coverage() {
                 cx.coverageinfo_finalize();
             }
 
-            // Create the llvm.used variable
+            // Create the llvm.compiler.used variable
             // This variable has type [N x i8*] and is stored in the llvm.metadata section
             if !cx.used_statics().borrow().is_empty() {
                 cx.create_used_variable()