[Support] Fix Windows directory_iterator_construct out of bounds

Fix incorrect use of the size of Path when accessing PathUTF16, as the
UTF-16 path can be shorter. Added unit test for coverage of this test
case.

Thanks to Ding Fei (danix800) for the code fix, see
https://reviews.llvm.org/D83321.

Differential Revision: https://reviews.llvm.org/D83689