clang-format: Make it very slighly more expensive to wrap between "= {".

This prevents unwanted fallout from r296664. Specifically in proto formatting,
this changed:
  optional Aaaaaaaa aaaaaaaa = 12 [
    (aaa) = aaaa,
    (bbbbbbbbbbbbbbbbbbbbbbbbbb) = {
      aaaaaaaaaaaaaaaaa: true,
      aaaaaaaaaaaaaaaa: true
    }
  ];

Into:
  optional Aaaaaaaa aaaaaaaa = 12 [
    (aaa) = aaaa,
    (bbbbbbbbbbbbbbbbbbbbbbbbbb) =
        {aaaaaaaaaaaaaaaaa: true, aaaaaaaaaaaaaaaa: true}
  ];

Which is considered less readable. Generally, it seems preferable to
format such dict literals as blocks rather than contract them to one
line.

llvm-svn: 297696