[SanitizerCoverage] Make Inline8bit and TracePC counters dead stripping resistant.

Summary:
Otherwise, at least on Mac, the linker eliminates unused symbols which
causes libFuzzer to error out due to a mismatch of the sizes of coverage tables.

Issue in Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=892167

Reviewers: morehouse, kcc, george.karpenkov

Reviewed By: morehouse

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D53113

llvm-svn: 344345