[SanitizerCoverage] Don't create comdat for interposable functions.

Summary:
Comdat groups override weak symbol behavior, allowing the linker to keep
the comdats for weak symbols in favor of comdats for strong symbols.

Fixes the issue described in:
https://bugs.chromium.org/p/chromium/issues/detail?id=918662

Reviewers: eugenis, pcc, rnk

Reviewed By: pcc, rnk

Subscribers: smeenai, rnk, bd1976llvm, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D56516

llvm-svn: 351247
diff --git a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
index 729197f..0ba8d57 100644
--- a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
+++ b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
@@ -568,7 +568,7 @@
       *CurModule, ArrayTy, false, GlobalVariable::PrivateLinkage,
       Constant::getNullValue(ArrayTy), "__sancov_gen_");
 
-  if (TargetTriple.supportsCOMDAT())
+  if (TargetTriple.supportsCOMDAT() && !F.isInterposable())
     if (auto Comdat =
             GetOrCreateFunctionComdat(F, TargetTriple, CurModuleUniqueId))
       Array->setComdat(Comdat);