Change the error message from "export declaration can only be used
within a module purview" to "export declaration can only be used within
a module interface" to be technically accurate.
The previous message was misleading because export declarations are
actually within a module purview when used in module implementation
units, but they are only allowed in module interface units.
This addresses the issue pointed out in GitHub issue #149008 where
Bigcheese noted that the diagnostic wording was incorrect.
Fixes#149008
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.
