[libSyntax] Add a reference counted version of OwnedString

We cannot use unowned strings for token texts of incrementally parsed
syntax trees since the source buffer to which reused nodes refer will
have been freed for reused nodes. Always copying the token text whenever
OwnedString is passed is too expensive. A reference counted copy of the
string allows us to keep the token's string alive across incremental
parses while eliminating unnecessary copies.

lib/Parse/Parser.cpp

@@ -312,15 +312,15 @@ swift::tokenizeWithTrivia(const LangOptions &LangOpts, const SourceManager &SM,
   syntax::AbsolutePosition RunningPos;
 
   tokenize(
-      LangOpts, SM, BufferID, Offset, EndOffset,
-      Diags,
+      LangOpts, SM, BufferID, Offset, EndOffset, Diags,
       CommentRetentionMode::AttachToNextToken, TriviaRetentionMode::WithTrivia,
       /*TokenizeInterpolatedString=*/false,
       /*SplitTokens=*/ArrayRef<Token>(),
       [&](const Token &Tok, const Trivia &LeadingTrivia,
           const Trivia &TrailingTrivia) {
+        auto Text = OwnedString::makeRefCounted(Tok.getText());
         auto ThisToken =
-            RawSyntax::make(Tok.getKind(), Tok.getText(), LeadingTrivia.Pieces,
+            RawSyntax::make(Tok.getKind(), Text, LeadingTrivia.Pieces,
                             TrailingTrivia.Pieces, SourcePresence::Present);
 
         auto ThisTokenPos = ThisToken->accumulateAbsolutePosition(RunningPos);