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/Format.cpp b/clang/lib/Format/Format.cpp
index 5807db9..5c7ce85 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -766,6 +766,7 @@
     GoogleStyle.JavaScriptWrapImports = false;
   } else if (Language == FormatStyle::LK_Proto) {
     GoogleStyle.AllowShortFunctionsOnASingleLine = FormatStyle::SFS_None;
+    GoogleStyle.AlwaysBreakBeforeMultilineStrings = false;
     GoogleStyle.SpacesInContainerLiterals = false;
     GoogleStyle.Cpp11BracedListStyle = false;
     // This affects protocol buffer options specifications and text protos.