[clang][modules] Disallow importing private framework in the implementation

Whenever we are compiling implementation of a framework (with the `-fmodule-name=FW` option), we never translate `#import <FW/Header.h>` to an import, regardless of whether "Header.h" belongs to "FW" or "FW_Private". For the same reasons, we also disallow `@import FW`. However, we still allow `@import FW_Private`. This patch disallows that a well, to be consistent with the rest of the rules.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D142167