[libcxx] Explicitly return the expected error code in create_directories if...

[libcxx] Explicitly return the expected error code in create_directories if the parent isn't a directory

On windows, going ahead and actually trying to create the directory
doesn't return an error code that maps to
std::errc::not_a_directory in this case.

This fixes two cases of
    TEST_CHECK(ErrorIs(ec, std::errc::not_a_directory))
in filesystems/fs.op.funcs/fs.op.create_directories/create_directories.pass.cpp
for windows (in testcases added in 59c72a70).

Differential Revision: https://reviews.llvm.org/D97090