c++: ICE with nested default targ lambdas [PR119574]

c++: ICE with nested default targ lambdas [PR119574]

In GCC 14 we fixed PR116567 in a more conservative way that doesn't
distinguish between the two kinds of deferred substitutions, and so
for PR119574 we instead ICE from get_innermost_template_args due to
TMPL_PARMS_DEPTH of the lambda, 2, being greater than the depth of the
augmented args, 1.

This patch works around the ICE in a best effort kind of way by guarding
the get_innermost_template_args call appropriately; I don't think it's
possible to get this completely right in GCC 14 without backporting the
proper fix for PR116567.

Note that lambda-targ13b.C present in the GCC 15 version of this patch[1]
never worked in GCC 14, and still doesn't work, which is why it's not
present in this patch.

[1]: r15-9350-gf3862ab07943d1

	PR c++/119574

gcc/cp/ChangeLog:

	* pt.cc (tsubst_lambda_expr): Don't call
	get_innermost_template_args if we're requesting too many
	levels.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ13.C: New test.
	* g++.dg/cpp2a/lambda-targ13a.C: New test.