[flang][runtime] Support all non-ASCII characters in temporary path on Windows

If the path to the TEMP folder contains (non-ASCII) characters that cannot be
encoded in the current 8-bit locale of the user, openfile_mkstemp might fail
on Windows.
That is an unlikely scenario. But given that the path to the default TEMP
folder on Windows contains the Windows user name, it is still possible.

Use the wide character Windows API to avoid that (unlikely) issue.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D151571