[include-fixer] Work around partial names in both directions.

We already handled the case where we had a nested name specifier where
parts from the beginning don't get a callback, also handle the case
where the end doesn't get a callback. This happens with function calls
in unknown namespaces.

The way we do this for now based on character data is a bit of a hack, we
may need to refine this later or fix clang to produce different callbacks.

llvm-svn: 269029