Expand parsing of braced init lists.

Allows formatting of C++11 braced init list constructs, like:
vector<int> v { 1, 2, 3 };
f({ 1, 2 });

This involves some changes of how tokens are handled in the
UnwrappedLineFormatter. Note that we have a plan to evolve the
design of the token flow into one where we create all tokens
up-front and then annotate them in the various layers (as we
currently already have to create all tokens at once anyway, the
current abstraction does not help). Thus, this introduces
FIXMEs towards that goal.