c++: lambda convop in C++23 [PR114632]

c++: lambda convop in C++23 [PR114632]

The lambda conversion was ICEing for two C++23 features, static op() and
explicit object parameters.  The issue with the former seems like a more
general issue: tsubst_function_decl recursing to substitute the parameters
was affected by cp_unevaluated_operand from the decltype that refers to the
declaration.  Various places already make a point of clearing
cp_unevaluated_operand ahead of PARM_DECL tsubsting; doing it here makes the
PR101233 fix redundant.

For explicit object lambdas, we want to implement CWG2561 and
just not declare the conversion.

	PR c++/114632
	PR c++/101233

gcc/cp/ChangeLog:

	* lambda.cc (maybe_add_lambda_conv_op): Not for xobj lambda.
	* pt.cc (tsubst_function_decl): Add cp_evaluated.
	(alias_ctad_tweaks): Revert PR101233 fix.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-lambda18.C: New test.
	* g++.dg/cpp23/static-operator-call7.C: New test.