Revamp the formatting of C++11 braced init lists.

The fundamental concept is:
Format as if the braced init list was a function call (with parentheses
replaced by braces). If there is no name/type before the opening brace
(e.g. if the braced list is nested), assume a zero-length identifier
just before the opening brace.

This behavior is gated on a new style flag, which for now replaces the
SpacesInBracedLists style flag. Activate this style flag for Google
style to reflect recent style guide changes.