swift234

ClangImporter: Make sure Foundation is available when bridging NSString.

If Foundation wasn't ever actually loaded into the ASTContext, type
lowering would fail to find the NSString-to-String conversion functions.
Fix this by only bridging NSString when a module named "Foundation" can
be loaded.

This still isn't very precise, since the bridging functions could still
be missing, but it at least avoids a user-triggerable error.

<rdar://problem/15027448>

Swift SVN r9380