GCOV: Make sure that function idents in the .gcda and .gcno match

When generating gcov compatible profiling, we sometimes skip emitting
data for functions for one reason or another. However, this was
emitting different function IDs in the .gcno and .gcda files, because
the .gcno case was using the loop index before skipping functions and
the .gcda the array index after. This resulted in completely invalid
gcov data.

This fixes the problem by making the .gcno loop track the ID
separately from the loop index.

llvm-svn: 221441