[sanitizer-coverage] add yet another flavour of coverage instrumentation: trace-pc-guard. The intent is to eventually replace all of {bool coverage, 8bit-counters, trace-pc} with just this one. Clang part

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281432 91177308-0d34-0410-b5e6-96231b3b80d8