[libcxx] Implement the stat function family on top of native windows APIs

While the windows CRTs (the modern UCRT, and the legacy msvcrt.dll
that mingw still often defaults to) do provide stat functions, they're
a bit lacking - they only provide second precision on the modification
time, lack support for symlinks and a few other details.

Instead reimplement them using a couple windows native functions,
getting exactly the info we need. (Technically, the implementation
within the CRT calls these functions anyway.)

If we only need a few fields, we could also do with fewer calls, as a
later optimization.

Differential Revision: https://reviews.llvm.org/D91141