Add triple to fix test failure.

This test uses `__regcall`, support for which is target-specific.

GitOrigin-RevId: f7c9de0de5804498085af973dc6bfc934a18f000
diff --git a/test/SemaCXX/attr-musttail.cpp b/test/SemaCXX/attr-musttail.cpp
index 55faf5a..561184e 100644
--- a/test/SemaCXX/attr-musttail.cpp
+++ b/test/SemaCXX/attr-musttail.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -verify -fsyntax-only -fms-extensions -fcxx-exceptions -fopenmp %s
+// RUN: %clang_cc1 -verify -fsyntax-only -fms-extensions -fcxx-exceptions -fopenmp -triple x86_64-linux %s
 
 int ReturnsInt1();
 int Func1() {