clang-format: Improve handling of unterminated string literals.

Before, clang-format would simply eat these as they were recognized as
whitespace. With this patch, they are mostly left alone.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186454 91177308-0d34-0410-b5e6-96231b3b80d8