update the identifier in case we've imported a definition of the macro (and
thus the contents of the header) from a module.

Also fold ExternalIdentifierLookup into ExternalPreprocessorSource; it no longer
makes sense to keep these separate now that the only user of the former also
needs the latter.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241137 91177308-0d34-0410-b5e6-96231b3b80d8