Start avoiding parse diagnostics on error tokens (#4431)

An invalid parse due to an error token isn't likely a great diagnostic
as it will already have been diagnosed by the lexer. A common case to
start handling that is when the parser encounters an invalid token when
expecting an expression.

This removes a number of unhelpful diagnostics after the lexer has done
a good job diagnosing.

This also means that there may be parse tree errors that aren't
diagnosed when there are lexer-diagnosed errors, so track that.

Follow-up to #4430 that almost finishes addressing its diagnostic TODO.

toolchain/parse/parse.cpp

@@ -45,7 +45,10 @@ auto Parse(Lex::TokenizedBuffer& tokens, DiagnosticConsumer& consumer,
   }
 
   context.AddLeafNode(NodeKind::FileEnd, *context.position());
-  tree.set_has_errors(err_tracker.seen_error());
+
+  // Mark the tree as potentially having errors if there were errors coming in
+  // from the tokenized buffer or we diagnosed new errors.
+  tree.set_has_errors(tokens.has_errors() || err_tracker.seen_error());
 
   if (auto verify = tree.Verify(); !verify.ok()) {
     // TODO: This is temporarily printing to stderr directly during development.