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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237397 91177308-0d34-0410-b5e6-96231b3b80d8