clang-format: Add special case for leading comments in braced lists.

A comment following the "{" of a braced list seems to almost always
refer to the first element of the list and thus should be aligned
to it.

Before (with Cpp11 braced list style):
  SomeFunction({  // Comment 1
                   "first entry",
                   // Comment 2
                   "second entry"});
After:
  SomeFunction({// Comment 1
                "first entry",
                // Comment 2
                "second entry"});

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197725 91177308-0d34-0410-b5e6-96231b3b80d8