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
