clang-format: Be more conservative about column layout formatting.

Specifically, if a braced list has at least one nested braced list,
format it either all on one line or in one column (i.e. one item per
line).

This seems in general to be an improvement as the structure of nested
braced lists can make a tightly packed outer braced list hard to read.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193345 91177308-0d34-0410-b5e6-96231b3b80d8