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;