c++: checking ICE w/ constexpr if and lambda as def targ [PR117054]

Here we're tripping over the assert in extract_locals_r which enforces
that an extra-args tree appearing inside another extra-args tree doesn't
actually have extra args.  This invariant doesn't always hold for lambdas
(which recently gained the extra-args mechanism) but that should be
harmless since cp_walk_subtrees doesn't walk LAMBDA_EXPR_EXTRA_ARGS and
so should be immune to the PR114303 issue for now.  So let's just disable
this assert for lambdas.

	PR c++/117054

gcc/cp/ChangeLog:

	* pt.cc (extract_locals_r): Disable tree_extra_args assert
	for LAMBDA_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ9.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>