clang-format: Add two new style options to support WebKit style.

New options:
* Break before the commas of constructor initializers and align
  the commas with the colon.
* Break before binary operators

Additionally, for styles without column limit, don't just accept
linebreaks done by the user, but instead remove 'invalid' (according
to the current style) linebreaks and add 'required' ones.

llvm-svn: 187210