[Coverage] Do not map regions from system headers

Do not assign source regions located within system headers file ID's,
and do not construct counter mapping regions out of them.

This makes coverage reports less cluttered and less mysterious. E.g
using the "assert" macro doesn't cause assert.h to appear in reports,
and it no longer shows the "assertion failed" branch as an uncovered
region.

It also makes coverage mapping sections a bit smaller (e.g a 1%
reduction in a stage2 build of bin/llvm-as).

llvm-svn: 275121