This patch addresses the review comments on r352930:
- Removes redundant diagnostic checking code
- Removes errnoneous use of diag::err_alias_is_definition, which
turned out to be ineffective anyway since functions can be defined later
in the translation unit and avoid detection.
- Adds a test for various invalid cases for import_name and import_module.
This reapplies D59520, with the addition of adding
`InGroup<IgnoredAttributes>` to the new warnings, to fix the
Misc/warning-flags.c failure.
Differential Revision: https://reviews.llvm.org/D59520
0d4e243456