[libcxx] [test] Don't look for the literal spelling __mbstate_t in warnings

[libcxx] [test] Don't look for the literal spelling __mbstate_t in warnings

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