[SanitizerCoverage] Add associated metadata to 8-bit counters.

[morehouse]

Summary

This allows counters associated with unused functions to be
dead-stripped along with their functions. This approach is the same one
we used for PC tables.

Fixes an issue where LLD removes an unused PC table but leaves the 8-bit
counter.

compiler-rt/test/fuzzer/gc-sections.test

 REQUIRES: linux, lld-available
 
 No gc-sections:
 RUN: %cpp_compiler %S/GcSectionsTest.cpp -o %t
 RUN: nm %t | grep UnusedFunctionShouldBeRemovedByLinker | count 1
 
-With gc-sections. Currently, we can't remove unused code.
+With gc-sections. Currently, we can't remove unused code except with LLD.
 RUN: %cpp_compiler %S/GcSectionsTest.cpp -o %t -fuse-ld=lld -ffunction-sections -Wl,-gc-sections
 RUN: nm %t | not grep UnusedFunctionShouldBeRemovedByLinker
+RUN: %run %t -runs=0 2>&1 | FileCheck %s
+CHECK-NOT: ERROR: The size of coverage PC tables does not match
 
 With gc sections, with trace-pc. Unused code is removed.
 RUN: %cpp_compiler %S/GcSectionsTest.cpp -o %t -fsanitize-coverage=0 -fsanitize-coverage=trace-pc -ffunction-sections -Wl,-gc-sections
 RUN: nm %t | not grep UnusedFunctionShouldBeRemovedByLinker

llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

void SanitizerCoverageModule::CreateFunctionLocalArrays(
   if (Options.TracePCGuard) {
     FunctionGuardArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int32Ty, SanCovGuardsSectionName);
     GlobalsToAppendToUsed.push_back(FunctionGuardArray);
   }
   if (Options.Inline8bitCounters) {
     Function8bitCounterArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int8Ty, SanCovCountersSectionName);
-    GlobalsToAppendToUsed.push_back(Function8bitCounterArray);
+    GlobalsToAppendToCompilerUsed.push_back(Function8bitCounterArray);
+    MDNode *MD = MDNode::get(F.getContext(), ValueAsMetadata::get(&F));
+    Function8bitCounterArray->addMetadata(LLVMContext::MD_associated, *MD);
   }
   if (Options.PCTable) {
     FunctionPCsArray = CreatePCArray(F, AllBlocks);
     GlobalsToAppendToCompilerUsed.push_back(FunctionPCsArray);
     MDNode *MD = MDNode::get(F.getContext(), ValueAsMetadata::get(&F));
     FunctionPCsArray->addMetadata(LLVMContext::MD_associated, *MD);
   }
 }