[flang][msvc] Define access flags under Windows. NFC.

[flang][msvc] Define access flags under Windows. NFC.

The flags F_OK, R_OK and W_OK are defined in unistd.h, which does not exist under the Windows platform. Windows still defines the `access` function. Its access flags are documented at https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess. For compatibility, define the flags F_OK, R_OK and W_OK using these constants.

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

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D88508