clang-format: Do not binpack initialization lists

Summary:
This patch tries to avoid binpacking when initializing lists/arrays, to allow things like:

  static int types[] = {
      registerType1(),
      registerType2(),
      registerType3(),
  };
  std::map<int, std::string> x = {
      { 0, "foo fjakfjaklf kljj" },
      { 1, "bar fjakfjaklf kljj" },
      { 2, "stuff fjakfjaklf kljj" },
  };

This is similar to how dictionnaries are formatted, and actually corresponds to the same conditions: when initializing a container (and not just 'calling' a constructor).

Such formatting involves 2 things:
* Line breaks around the content of the block. This can be forced by adding a comma or comment after the last element
* Elements should not be binpacked

This patch considers the block is an initializer list if it either ends with a comma, or follows an assignment, which seems to provide a sensible approximation.

Reviewers: krasimir, djasper

Reviewed By: djasper

Subscribers: malcolm.parsons, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D34238

llvm-svn: 306868