clang-format: Be more conservative about braced list column layout.

Specifically disable it for nested braced lists as it commonly can look
really weird. Eventually, we'll want to become smarter and format some of
the nested lists better.

Before:
  SomeStruct my_struct_array = {
    { aaaaaa,     aaaaaaaa,   aaaaaaaaaa, aaaaaaaaa, aaaaaaaaa,
      aaaaaaaaaa, aaaaaaaaaa, aaaaaaa,    aaa },
    { aaaa, aaaa, aaaa, aaaa, aaaa, aaaa, aaaa, aaa },
    { aaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaa,
      aaaaaaaaaaaa,      a,                 aaaaaaaaaa,
      aaaaaaaaa,         aaa },
  };

After:
  SomeStruct my_struct_array = {
    { aaaaaa, aaaaaaaa, aaaaaaaaaa, aaaaaaaaa, aaaaaaaaa, aaaaaaaaaa,
      aaaaaaaaaaaa, aaaaaaa, aaa },
    { aaaa, aaaa, aaaa, aaaa, aaaa, aaaa, aaaa, aaa },
    { aaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaa,
      aaaaaaaaaaaa, a, aaaaaaaaaa, aaaaaaaaa, aaa },
  };

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196783 91177308-0d34-0410-b5e6-96231b3b80d8