c++: lambda in requires outside template [PR99546]

c++: lambda in requires outside template [PR99546]

Since r10-7441 we set processing_template_decl in a requires-expression so
that we can use tsubst_expr to evaluate the requirements, but that confuses
lambdas terribly; begin_lambda_type silently returns error_mark_node and we
continue into other failures.  This patch clears processing_template_decl
again while we're defining the closure and op() function, so it only remains
set while parsing the introducer (i.e. any init-captures) and building the
resulting object.  This properly avoids trying to create another lambda in
tsubst_lambda_expr.

	PR c++/99546
	PR c++/113925
	PR c++/106976
	PR c++/109961
	PR c++/117336

gcc/cp/ChangeLog:

	* lambda.cc (build_lambda_object): Handle fake
	requires-expr processing_template_decl.
	* parser.cc (cp_parser_lambda_expression): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-requires2.C: New test.
	* g++.dg/cpp2a/lambda-requires3.C: New test.
	* g++.dg/cpp2a/lambda-requires4.C: New test.
	* g++.dg/cpp2a/lambda-requires5.C: New test.

(cherry picked from commit 25992d8daff60726a247ec7850d540aed5335639)