Drop the Identifier List from Designated Type Parsing

This was never the correct way to model this because it drops
the commas in the list. Instead just take the optional trailing
identifier element if we have one.

Author: CodaFi

lib/Parse/ParseDecl.cpp

@@ -8863,8 +8863,6 @@ Parser::parseDeclOperatorImpl(SourceLoc OperatorLoc, Identifier Name,
       return makeParserCodeCompletionResult<OperatorDecl>();
     }
 
-    SyntaxParsingContext ListCtxt(SyntaxContext, SyntaxKind::IdentifierList);
-
     (void)parseIdentifier(groupName, groupLoc,
                           diag::operator_decl_expected_precedencegroup,
                           /*diagnoseDollarPrefix=*/false);