clang-format: Fix corner case for string splitting ..

.. in conjunction with Style.AlwaysBreakBeforeMultilineStrings. Also,
simplify the implementation by handling newly split strings and already
split strings by the same code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189102 91177308-0d34-0410-b5e6-96231b3b80d8