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.