c++: Fix lambda scope cleanup after class error [PR125343]

cp_parser_class_head opens a lambda mangling scope for a valid class
head.  If begin_class_definition rejects that type, it replaces it with
error_mark_node.  The old cleanup condition consequently leaves the
scope open, and lambda error recovery reaches
record_lambda_scope_sig_discriminator with the wrong active scope.

Remember whether the class head opened a lambda scope and use that
state for cleanup independently of whether the definition succeeds.

	PR c++/125343

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_class_specifier): Preserve whether the
	class head opened a lambda scope and use it for cleanup.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/pr125343.C: New test.

Signed-off-by: Odysseas Georgoudis <odygrd@hotmail.com>