[ODRHash] Fix early exit that skipped code.

There is a bit of code at the end of AddDeclaration that should be run on
every exit of the function.  However, there was an early exit beforehand
that could be triggered, which causes a small amount of data to skip the
hashing, leading to false positive mismatch.  Use a separate function so
that this code is always run.

llvm-svn: 342199