InstrProf: Avoid creating profile names for symbols in system headers

We don't emit any coverage mapping for uncovered functions that come
from system headers, but we were creating a GlobalVariable with each
of their names. This is wasteful since the linker will need to dead
strip the unused symbols, and it can lead to issues when merging
coverage with others TUs that do have coverage for those functions.

llvm-svn: 226764