Fix test failure introduced in ab187bb

This patch, #147285 introduced a test that used
preserve_none/preserve_all/preserve_most for the purposes of validating
its ast-dump.  However, this attribute isn't supported on some
platforms, so this patch adds a triple to the test to ensure we try to
run it as a non-supported platform.

As this is simply an ast-dump test, there is no value to running it on
other configurations.

Author: erichkeane

clang/test/AST/ast-print-cconv-preserve.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -ast-print %s -o - | FileCheck %s
+// RUN: %clang_cc1 -ast-print -triple x86_64-unknown-unknown %s -o - | FileCheck %s
 
 void (__attribute__((preserve_none)) *none)();