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
diff --git a/clang/lib/Format/FormatTokenLexer.cpp b/clang/lib/Format/FormatTokenLexer.cpp
index 199d297..284a3a2 100644
--- a/clang/lib/Format/FormatTokenLexer.cpp
+++ b/clang/lib/Format/FormatTokenLexer.cpp
@@ -691,7 +691,9 @@
     }
   }
 
-  if (Style.Language == FormatStyle::LK_JavaScript &&
+  if ((Style.Language == FormatStyle::LK_JavaScript ||
+       Style.Language == FormatStyle::LK_Proto ||
+       Style.Language == FormatStyle::LK_TextProto) &&
       Tok.is(tok::char_constant)) {
     Tok.Tok.setKind(tok::string_literal);
   }