Order initializers of static data members of explicit specializations

I tried to implement this properly in r189051, but I didn't have enough
test coverage.  Richard kindly provided more test cases than I could
possibly imagine and now we should have the correct condition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189898 91177308-0d34-0410-b5e6-96231b3b80d8