[SanitizerCoverage] Create comdat for global arrays.

Summary:
Place global arrays in comdat sections with their associated functions.
This makes sure they are stripped along with the functions they
reference, even on the BFD linker.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: eraman, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D51902

llvm-svn: 341951