Format unions like structs and classes.

Note that I don't know whether we should put {} on a single line in this
case, but it is probably a theoretical issue as in practice such
structs, classes or unions won't be empty.

Before: union A {}
        a;
After:  union A {} a;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172355 91177308-0d34-0410-b5e6-96231b3b80d8