Formalize some SourceFile parsing outputs

Currently when parsing a SourceFile, the parser
gets handed pointers so that it can write the
interface hash and collected tokens directly into
the file. It can also call `setSyntaxRoot` at
the end of parsing to set the syntax tree.

In preparation for the removal of
`performParseOnly`, this commit formalizes these
values as outputs of `ParseSourceFileRequest`,
ensuring that the file gets parsed when the
interface hash, collected tokens, or syntax tree
is queried.

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -719,9 +719,7 @@ public:
   }
 
   void parse() {
-    auto root = Parser->parse();
-    if (SynTreeCreator)
-      SynTreeCreator->acceptSyntaxRoot(root, Parser->getSourceFile());
+    Parser->parse();
   }
 
   SourceFile &getSourceFile() {