[PGO] Simplify coverage mapping lowering

[PGO] Simplify coverage mapping lowering

Coverage mapping data may reference names of functions
that are skipped by FE (e.g, unused inline functions). Since
those functions are skipped, normal instr-prof function lowering
pass won't put those names in the right section, so special
handling is needed to walk through coverage mapping structure
and recollect the references.

With this patch, only names that are skipped are processed. This
simplifies the lowering code and it no longer needs to make
assumptions coverage mapping data layout. It should also be
more efficient.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@257092 91177308-0d34-0410-b5e6-96231b3b80d8