[JSONSerialization] Introduce ScalarReferenceTraits

For ScalarTraits, a buffer was always created on the heap to which the
scalar string value was written just to be copied to the output buffer
again. In case the value already exists in a memory buffer it is way
cheaper to avoid the heap allocation and copy it straight to the output
buffer.

include/swift/Syntax/Serialization/SyntaxSerialization.h

@@ -33,20 +33,39 @@ static void *DontSerializeNodeIdsUserInfoKey = &DontSerializeNodeIdsUserInfoKey;
 
 /// Serialization traits for SourcePresence.
 template <>
-struct ScalarEnumerationTraits<syntax::SourcePresence> {
-  static void enumeration(json::Output &out, syntax::SourcePresence &value) {
-    out.enumCase(value, "Present", syntax::SourcePresence::Present);
-    out.enumCase(value, "Missing", syntax::SourcePresence::Missing);
+struct ScalarReferenceTraits<syntax::SourcePresence> {
+  static StringRef stringRef(const syntax::SourcePresence &value) {
+    switch (value) {
+    case syntax::SourcePresence::Present:
+      return "\"Present\"";
+    case syntax::SourcePresence::Missing:
+      return "\"Missing\"";
+    }
+  }
+
+  static bool mustQuote(StringRef) {
+    // The string is already quoted. This is more efficient since it does not
+    // check for characters that need to be escaped
+    return false;
   }
 };
 
 /// Serialization traits for swift::tok.
 template <>
-struct ScalarEnumerationTraits<tok> {
-  static void enumeration(Output &out, tok &value) {
+struct ScalarReferenceTraits<tok> {
+  static StringRef stringRef(const tok &value) {
+    switch (value) {
 #define TOKEN(name) \
-    out.enumCase(value, #name, tok::name);
+    case tok::name: return "\"" #name "\"";
 #include "swift/Syntax/TokenKinds.def"
+    default: llvm_unreachable("Unknown token kind");
+    }
+  }
+
+  static bool mustQuote(StringRef) {
+    // The string is already quoted. This is more efficient since it does not
+    // check for characters that need to be escaped
+    return false;
   }
 };