InstrProf: Only disable coverage in built-in macros, not all system macros

The issue I was trying to solve in r236547 was about built-in macros,
but I disabled coverage in all system macros. This is actually a bit
of overkill, and makes the display of coverage around system macros
degrade unnecessarily. Instead, limit this to builtins specifically.

llvm-svn: 237397