c++: Allow template lambdas without lambda-declarator [PR97839]

c++: Allow template lambdas without lambda-declarator [PR97839]

Our implementation of template lambdas incorrectly requires the optional
lambda-declarator.  This was probably required by an early draft of
generic lambdas, but now the production is [expr.prim.lambda.general]:

 lambda-expression:
    lambda-introducer lambda-declarator [opt] compound-statement
    lambda-introducer < template-parameter-list > requires-clause [opt]
	  lambda-declarator [opt] compound-statement

Therefore, we should accept the following test.

gcc/cp/ChangeLog:

	PR c++/97839
	* parser.c (cp_parser_lambda_declarator_opt): Don't require ().

gcc/testsuite/ChangeLog:

	PR c++/97839
	* g++.dg/cpp2a/lambda-generic8.C: New test.