InstrProf: Handle unknown functions if they consist only of zero-regions

InstrProf: Handle unknown functions if they consist only of zero-regions

This comes up when we generate coverage for a function but don't end
up emitting the function at all - dead static functions or inline
functions that aren't referenced in a particular TU, for example. In
these cases we'd like to show that the function was never called,
which is trivially true.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229717 91177308-0d34-0410-b5e6-96231b3b80d8