[sanitizer] Split coverage into separate RT in sanitizer_common

Summary:
`sanitizer_common`'s coverage support is fairly well separated, and libcdep by
default. Several sanitizers don't make use of coverage, and as far as I can
tell do no benefit from the extra dependencies pulled in by the coverage public
interface functions.

The following sanitizers call `InitializeCoverage` explicitely: MSan, ASan,
LSan, HWAsan, UBSan. On top of this, any sanitizer bundling RTUBSan should
add the coverage RT as well: ASan, Scudo, UBSan, CFI (diag), TSan, MSan, HWAsan.

So in the end the following have no need: DFSan, ESan, CFI, SafeStack (nolibc
anyway), XRay, and the upcoming Scudo minimal runtime.

I tested this with all the sanitizers check-* with gcc & clang, and in
standalone on Linux & Android, and there was no issue. I couldn't test this on
Mac, Fuchsia, BSDs, & Windows for lack of an environment, so adding a bunch of
people for additional scrunity. I couldn't test HWAsan either.

Reviewers: eugenis, vitalybuka, alekseyshl, flowerhack, kubamracek, dberris, rnk, krytarowski

Reviewed By: vitalybuka, alekseyshl, flowerhack, dberris

Subscribers: mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D44701

llvm-svn: 328204

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