c++: Implement P1102R2 - Down with ()!

c++: Implement P1102R2 - Down with ()!

The following patch implements P1102R2.
For attributes, we have already attribute parsing before the parameter
declarations and so when that is omitted, if the attributes are first we
already accept it.

2021-02-26  Jakub Jelinek  <jakub@redhat.com>

	* parser.c (cp_parser_lambda_declarator_opt): Implement
	P1102R2 - Down with ()! Make ()s optional before lambda specifiers
	for -std={c,gnu}++2b or with pedwarn in earlier versions.

	* g++.dg/cpp23/lambda-specifiers1.C: New test.