swift234

Clang importer: handle enumerator prefix stripping in importFullName().

Centralize the mapping of C names to Swift names further by including
enumerator prefix stripping, rather than having that as a separate
path. The actual logic and code for computing the prefix is unchanged
(despite moving from one file to another). This corrects the name
computed for the Swift lookup tables, but is an NFC refactoring for
everything else.

With this, kill off importName(), because it's been entirely
superseded by importFullName().