Fix volatile _Complex alignment test on platforms where 64-bit floating point isn't 64-bit aligned

Add x86-64 triple, and check its datalayout. Also add some comments, and use the new CHECK-LABEL.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@186564 91177308-0d34-0410-b5e6-96231b3b80d8