Fix failing test case for _Countof

Test just needs an explicit triple that was missed.

Author: AaronBallman

clang/test/C/C2y/n3369_2.c

@@ -1,5 +1,5 @@
 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 5
-// RUN: %clang_cc1 -std=c2y -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -std=c2y -triple x86_64-unknown-unknown -emit-llvm -o - %s | FileCheck %s
 
 // This tests the codegen behavior for _Countof.
 // CHECK-LABEL: define dso_local i32 @test1(