While the code uses the type name `std::mbstate_t`, the warning message
mentions the original underlying type, which is a C library internal
type name.
On Windows this type is called `_Mbstatet` instead of `__mbstate_t`. Use
expect-warning-re to avoid spelling out the literal name of the type.
Due to issues with the detection of the clang-verify feature, these
tests have been skipped in the Windows CI configuration so far.
Differential Revision: https://reviews.llvm.org/D103309
b48a256e9c