Improve formatting of multi-variable DeclStmts.

This fixed llvm.org/PR15670

Before:
aaaaaaaaa a = aaaaaaaaaaaaaaaaaaaa, b = bbbbbbbbbbbbbbbbbbbb,
                                    c = cccccccccccccccccccc,
                                    d = dddddddddddddddddddd;
aaaaaaaaa *a = aaaaaaaaaaaaaaaaaaa, *b = bbbbbbbbbbbbbbbbbbb,
                                     *c = ccccccccccccccccccc,
                                      *d = ddddddddddddddddddd;

After:
aaaaaaaaa a = aaaaaaaaaaaaaaaaaaaa, b = bbbbbbbbbbbbbbbbbbbb,
          c = cccccccccccccccccccc, d = dddddddddddddddddddd;
aaaaaaaaa *a = aaaaaaaaaaaaaaaaaaa, *b = bbbbbbbbbbbbbbbbbbb,
          *c = ccccccccccccccccccc, *d = ddddddddddddddddddd;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178860 91177308-0d34-0410-b5e6-96231b3b80d8