Implement -sanitizer-coverage-gated-trace-callbacks to gate the
invocation of the tracing callbacks based on the value of a global
variable, which is stored in a specific section.
When this option is enabled, the instrumentation will not call into the
runtime-provided callbacks for tracing, thus only incurring in a trivial
branch without going through a function call. It is up to the runtime to
toggle the value of the global variable in order to enable tracing.
This option is only supported for trace-pc-guard.
Note: will add additional support for trace-cmp in a follow up PR.
Patch by Filippo Bigarella
rdar://101626834
