swift234

[Clang importer] Cache enum information based on modulename.enumname.

Caching information based on Clang declaration pointers, when that
information is used via importFullName, is unsafe because the
declaration pointers may point into Clang AST contexts built
temporarily built for modules, and therefore go stale. I believe this
is the cause of the recent ASan failures.

Again, the whole passing-through-a-different-Clang-Sema thing with
importFullName is an architectural fail that requires refactoring, so
we don't keep falling into this trap.