CodeGen: Include a function hash in instrumentation based profiling

The hash itself is still the number of counters, which isn't all that
useful, but this separates the API changes from the actual
implementation of the hash and will make it easier to transition to
the ProfileData library once it's implemented.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204186 91177308-0d34-0410-b5e6-96231b3b80d8