move GetOrCreateFunctionComdat to Instrumentation.cpp/Instrumentation.h

Summary:
GetOrCreateFunctionComdat is currently used in SanitizerCoverage,
where it's defined. I'm planing to use it in HWASAN as well,
so moving it into a common location.
NFC

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53218

llvm-svn: 344433