[clang-tidy] Fix test that requires Windows platofrm

[jubnzv]

Summary

This commit fixes the cppcoreguidelines-pro-type-vararg test when it runs on a Windows host, but the toolchain is targeted a non-Windows platform.
See the description of the problem with this test here: https://reviews.llvm.org/D101259#2739466

[njames93]

LGTM.

clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-pro-type-vararg-ms.cpp

 // Purpose:
 // Ensure that the 'cppcoreguidelines-pro-type-vararg' check works with the
 // built-in va_list on Windows systems.
 
-// REQUIRES: system-windows
-
-// RUN: %check_clang_tidy %s cppcoreguidelines-pro-type-vararg %t
+// RUN: %check_clang_tidy %s cppcoreguidelines-pro-type-vararg %t -- --extra-arg=--target=x86_64-windows
 
 void test_ms_va_list(int a, ...) {
   __builtin_ms_va_list ap;
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: do not declare variables of type va_list; use variadic templates instead
   __builtin_ms_va_start(ap, a);
   int b = __builtin_va_arg(ap, int);
   // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: do not use va_arg to define c-style vararg functions; use variadic templates instead
   __builtin_ms_va_end(ap);