clang-format: Prevent column layout if elements aren't uniform enough.

This patch only considers the difference between the length of the
shortest and longest element, but we might want to look at other
features (token count, etc.) in future.

Before:
  std::vector<MyValues> aaaaaaaaaaaaaaaaaaa{
      aaaaaaa,      aaaaaaaaaa,
      aaaaa,        aaaaaaaaaaaaaaa,
      aaa,          aaaaaaaaaa,
      a,            aaaaaaaaaaaaaaaaaaaaa,
      aaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaa + aaaaaaaaaaaaaaaaaaa,
      aaaaaaa,      a};

After:
  std::vector<MyValues> aaaaaaaaaaaaaaaaaaa{
      aaaaaaa, aaaaaaaaaa, aaaaa, aaaaaaaaaaaaaaa, aaa, aaaaaaaaaa, a,
      aaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaa,
      aaaaaaaaaaaaaaaaaaa + aaaaaaaaaaaaaaaaaaa, aaaaaaa, a};

llvm-svn: 218111