[Flang] Fix error messages on Windows.

Flang uses positional arguments for `messages::say()`, such as "%1$s" which is only supported in MS Compilers with the `_*printf_p` form of the function. This uses a conditional macro to convert the existing `vsnprintf` used to the one needed in MS-World.

7 tests in D107575 rely on this change.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D107654

flang/lib/Parser/message.cpp

@@ -38,14 +38,26 @@ void MessageFormattedText::Format(const MessageFixedText *text, ...) {
   }
   va_list ap;
   va_start(ap, text);
+#ifdef _MSC_VER
+  // Microsoft has a separate function for "positional arguments", which is
+  // used in some messages.
+  int need{_vsprintf_p(nullptr, 0, p, ap)};
+#else
   int need{vsnprintf(nullptr, 0, p, ap)};
+#endif
+
   CHECK(need >= 0);
   char *buffer{
       static_cast<char *>(std::malloc(static_cast<std::size_t>(need) + 1))};
   CHECK(buffer);
   va_end(ap);
   va_start(ap, text);
+#ifdef _MSC_VER
+  // Use positional argument variant of printf.
+  int need2{_vsprintf_p(buffer, need + 1, p, ap)};
+#else
   int need2{vsnprintf(buffer, need + 1, p, ap)};
+#endif
   CHECK(need2 == need);
   va_end(ap);
   string_ = buffer;