Previously we forbiden the users to import named modules from clang header
modules. However, due to an oversight, the @import form of Objective C
got involved. This is not want and we fix that in this patch.
Close https://github.com/llvm/llvm-project/issues/64755
This wouldn't affect the form @import as the test shows. The two
affected test case `diag-flags.cpp` and `diag-pragma.cpp` are old test
cases in 2017 and 2018, when we're not so clear about the direction of
modules. And the things that these 2 tests tested can be covered by
clang modules naturally. So I change the them into clang modules to
not block this patch.