Formatter: Initial support for ObjC dictionary literals.

Before:
  @{
  foo:
    bar
  }
  ;

Now:
  @{ foo : bar };

parseBracedList() already does the right thing from an UnwrappedLineParser
perspective, so check for "@{" in all loops that process constructs that can
contain expressions and call parseBracedList() if found.

llvm-svn: 174840