P0428R2 - familiar template syntax for generic lambdas

	P0428R2 - familiar template syntax for generic lambdas
	* parser.c (cp_parser_lambda_declarator_opt): Don't pedwarn
	for cxx2a and above, reword pedwarn for C++14/C++17.

	* g++.dg/cpp1y/lambda-generic-x.C: Adjust warnings and limit
	to c++17_down target.
	* g++.dg/cpp1y/lambda-generic-dep.C: Likewise.
	* g++.dg/cpp1y/lambda-generic-77914.C: Adjust error and limit
	to c++17_down target.
	* g++.dg/cpp2a/lambda-generic1.C: New test.
	* g++.dg/cpp2a/lambda-generic2.C: New test.
	* g++.dg/cpp2a/lambda-generic3.C: New test.
	* g++.dg/cpp2a/lambda-generic4.C: New test.
	* g++.dg/cpp2a/lambda-generic5.C: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254991 138bc75d-0d04-0410-961f-82ee72b054a4