clang-format: Don't break lines starting with "import <string-literal>"

The author might be missing the "#" or these might be protocol buffer
definitions. Either way, we should not break the line or the string.

There don't seem to be other valid use cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199501 91177308-0d34-0410-b5e6-96231b3b80d8