swift234

Revert "[CMake] Clear the Clang module cache before building the ObjectiveC overlay."

This broke systems where it isn't appropriate to clear the module cache.
The build script already does this when you do a full build; for other
cases, we should just deal with it. (You can still specify a custom
MODULE_CACHE_PATH to make it easier to delete, rather than relying on the
system one.)

This reverts commit r14364.

<rdar://problem/16232176>

Swift SVN r14872