InstrProf: Simplify/reduce state in CoverageMapping (NFC)

This state object makes things harder to reason about and isn't really
useful, since we can just emit the mappings before the state changes
rather than holding on to it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224476 91177308-0d34-0410-b5e6-96231b3b80d8