[Syntax] Make getRaw() return const reference

This saves reference-counting operations.

Author: rintaro

include/swift/Syntax/Syntax.h

@@ -82,7 +82,7 @@ class Syntax {
   SyntaxKind getKind() const;
 
   /// Get the shared raw syntax.
-  RC<RawSyntax> getRaw() const;
+  const RC<RawSyntax> &getRaw() const;
 
   /// Get an ID for this node that is stable across incremental parses
   SyntaxNodeId getId() const { return getRaw()->getId(); }

include/swift/Syntax/SyntaxData.h

@@ -182,7 +182,7 @@ class SyntaxData final
                              CursorIndex IndexInParent = 0);
 
   /// Returns the raw syntax node for this syntax node.
-  const RC<RawSyntax> getRaw() const {
+  const RC<RawSyntax> &getRaw() const {
     return Raw;
   }
 

lib/Syntax/Syntax.cpp

@@ -17,7 +17,7 @@
 using namespace swift;
 using namespace swift::syntax;
 
-RC<RawSyntax> Syntax::getRaw() const {
+const RC<RawSyntax> &Syntax::getRaw() const {
   return Data->getRaw();
 }