The option -sanitizer-coverage-gated-trace-callbacks gates the
invocation of the trace-pc-guard callbacks based on the value of a
global variable, which is stored in a specific section.
In this commit, we extend this feature to trace-cmp and gate the cmp
callbacks to the same variable used for trace-pc-guard.
Update SanitizerCoverage doc with this flag.
rdar://135404160
Patch by: Andrea Fioraldi
7800d59f5b