[clang-format] Add SpaceBeforeCpp11BracedList option.

[clang-format] Add SpaceBeforeCpp11BracedList option.

WebKit C++ style for object initialization is as follows:

  Foo foo { bar };

Yet using clang-format -style=webkit changes this to:

  Foo foo{ bar };

As there is no existing combination of rules that will ensure a space
before a braced list in this fashion, this patch adds a new
SpaceBeforeCpp11BracedList rule.

Patch by Ross Kirsling!

Differential Revision: https://reviews.llvm.org/D46024

llvm-svn: 334692