[SanitizerCoverage] sancov/inline-bool-flag instrumentation.

[SanitizerCoverage] sancov/inline-bool-flag instrumentation.

Summary:
New SanitizerCoverage feature `inline-bool-flag` which inserts an
atomic store of `1` to a boolean (which is an 8bit integer in
practice) flag on every instrumented edge.

Implementation-wise it's very similar to `inline-8bit-counters`
features. So, much of wiring and test just follows the same pattern.

Reviewers: kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, hiraditya, jfb, cfe-commits, #sanitizers

Tags: #clang, #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D77244