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

[Meinersbur]

Summary

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.

flang/runtime/file.cpp

 int OpenFile::PendingResult(const Terminator &terminator, int iostat) {
   int id{nextId_++};
   pending_ = New<Pending>{terminator}(id, iostat, std::move(pending_));
   return id;
 }
 
 bool IsATerminal(int fd) { return ::isatty(fd); }
 
+#ifdef WIN32
+// Access flags are normally defined in unistd.h, which unavailable under
+// Windows. Instead, define the flags as documented at
+// https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess
+#define F_OK 00
+#define W_OK 02
+#define R_OK 04
+#endif
+
 bool IsExtant(const char *path) { return ::access(path, F_OK) == 0; }
 bool MayRead(const char *path) { return ::access(path, R_OK) == 0; }
 bool MayWrite(const char *path) { return ::access(path, W_OK) == 0; }
 bool MayReadAndWrite(const char *path) {
   return ::access(path, R_OK | W_OK) == 0;
 }
 } // namespace Fortran::runtime::io