c++: unifying LAMBDA_EXPR [PR122318]

c++: unifying LAMBDA_EXPR [PR122318]

This patch makes the default case of unify accept LAMBDA_EXPR P/A pairs,
which we can legitimately hit during alias CTAD guide overload resolution.
We can also less legimitately hit this during partial specialization
matching (likely IFNDR).

I'm not totally sure if we want to handle them like any other non-deducible
expression vs handling them separately (returning success iff they're ==).
I couldn't come up with a testcase for which it mattered so I went the
simpler route.

	PR c++/122318
	PR c++/101670

gcc/cp/ChangeLog:

	* pt.cc (unify) <default>: Accept LAMBDA_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ20.C: New test.
	* g++.dg/cpp2a/lambda-targ21.C: New test.

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