CodeGen: Improve warnings about uninstrumented files when profiling

Improve the warning when building with -fprofile-instr-use and a file
appears not to have been profiled at all. This keys on whether a
function is defined in the main file or not to avoid false negatives
when one includes a header with functions that have been profiled.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211760 91177308-0d34-0410-b5e6-96231b3b80d8