[flang][msvc] Use platform-independent primitives in temporary f18 driver.

Use functions for process launching, temporary file creation and file deletion from LLVM support library instead of POSIX-specific ones.

This patch is part of the series to [[ http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html | make flang compilable with MS Visual Studio ]].

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D85660