[SanitizerCoverage] Add associated metadata to pc-tables.

Summary:
Using associated metadata rather than llvm.used allows linkers to
perform dead stripping with -fsanitize-coverage=pc-table.  Unfortunately
in my local tests, LLD was the only linker that made use of this metadata.

Partially addresses https://bugs.llvm.org/show_bug.cgi?id=34636 and fixes
https://github.com/google/sanitizers/issues/971.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: Dor1s, hiraditya, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D48203

llvm-svn: 334858