clang-format: Properly handle implicit string concatenation in text protos

Three issues to fix:
- char_constants weren't properly treated as string literals
- Prevening the break after "label: " does not make sense in concunction
  with AlwaysBreakBeforeMultilineStrings. It leads to situations where
  clang-format just cannot find a viable format (it must break and yet
  it must not break).
- AlwaysBreakBeforeMultilineStrings should not be on for LK_TextProto in
  Google style.

llvm-svn: 327255