[ADT] Use a nonce type with at least 4 byte alignment.

We didn't actually statically check this, and so it worked 25% of the
time for me. =/ Really sorry it took so long to fix, I shouldn't leave
the commit log editor window open without saving and landing the commit.
=[

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256528 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/ADT/PointerIntPairTest.cpp

@@ -16,6 +16,7 @@ namespace {
 
 TEST(PointerIntPairTest, GetSet) {
   struct S {
+    int i;
   };
   S s;