[Parser] Decouple the parser from AST creation (part 1)

Instead of creating the AST directly in the parser (and libSyntax or
SwiftSyntax via SyntaxParsingContext), make Parser to explicitly create
a tree of ParsedSyntaxNodes. Their OpaqueSyntaxNodes can be either
libSyntax or SwiftSyntax. If AST is needed, it can be generated from the
libSyntax tree.

tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp

@@ -155,6 +155,7 @@ private:
     node.present = true;
   }
 
+public:
   OpaqueSyntaxNode recordToken(tok tokenKind,
                                ArrayRef<ParsedTriviaPiece> leadingTrivia,
                                ArrayRef<ParsedTriviaPiece> trailingTrivia,
@@ -201,6 +202,10 @@ private:
     auto result = NodeLookup(lexerOffset, ckind);
     return {result.length, result.node};
   }
+
+  OpaqueSyntaxNodeKind getOpaqueKind() override {
+    return OpaqueSyntaxNodeKind::SwiftSyntax;
+  }
 };
 
 static swiftparser_diagnostic_severity_t getSeverity(DiagnosticKind Kind) {