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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351247 91177308-0d34-0410-b5e6-96231b3b80d8