Finish Parser2 support and switch. (#2381)

Adds remaining expression support, and switches the default to Parser2.

Note, this doesn't delete the current Parser yet. I'll just do that in its own PR.

toolchain/parser/parse_tree.cpp

@@ -28,9 +28,7 @@ auto ParseTree::Parse(TokenizedBuffer& tokens, DiagnosticConsumer& consumer)
   TokenDiagnosticEmitter emitter(translator, consumer);
 
   // Delegate to the parser.
-  // TODO: Edit this to swap between Parser and Parser2. This is manual in order
-  // to avoid test duplication.
-  return Parser::Parse(tokens, emitter);
+  return Parser2::Parse(tokens, emitter);
 }
 
 auto ParseTree::postorder() const -> llvm::iterator_range<PostorderIterator> {