Slightly modify the formatting rules for braced lists.

Basically, the new rule is: The opening "{" always has to be on the
same line as the first element if the braced list is nested
(e.g. in another braced list or in a function).

The solution that clang-format produces almost always adheres to this
rule anyway and this makes clang-format significantly faster for larger
lists. Added a test cases for the only exception I could find
(which doesn't seem to be very important at first sight).