stdint.h and stddef.h are shipped with CLang, but they not included in
Clang's module.map, which causes Clang to import libc versions instead
(and Clang's stdint.h is dispatching to libc). This was causing
hard-to-debug transient failures during incremental rebuilds, like this:
error: module file was created by an older version of the compiler: .../Darwin.swiftmodule
Swift SVN r23230
