[test] alias-unused.cpp: set triple to Linux

Otherwise the test would fail on Darwin and other platforms that use
Itanium ABI but do not support alias/ifunc.

Author: MaskRay

clang/test/Sema/alias-unused.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -triple %itanium_abi_triple -Wunused -x c -verify %s
-// RUN: %clang_cc1 -triple %itanium_abi_triple -Wunused -verify=expected,cxx %s
+// RUN: %clang_cc1 -triple x86_64-linux -Wunused -x c -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux -Wunused -verify=expected,cxx %s
 
 #ifdef __cplusplus
 extern "C" {