clang-format: Handle trailing commas in column layout of braced list.

Before, this was causing errors.

Also exit early in breakProtrudingToken() (before the expensive call to
SourceManager::getSpellingColumnNumber()). This makes formatting huge
(100k+-item) braced lists possible.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@189094 91177308-0d34-0410-b5e6-96231b3b80d8