c++: generic lambdas and local-externs from outer scopes [PR 99030]

c++: generic lambdas and local-externs from outer scopes [PR 99030]

Lambdas can refer to local externs from their enclosing scope.  When
the lambda's generic but the containing function is not a temploid,
we'll never have tsubsted the declaring decl so won't have a local
specialization.  But in that case we can just use the decl we
tsubsting directly -- it's not dependent.

	PR c++/99030
	gcc/cp
	* pt.c (tsubst_copy) [VAR_DECL]: For a DECL_LOCAL_DECL_P T is the
	answer if there's no local specialization.
	gcc/testsuite/
	* g++.dg/lookup/pr99030.C: New.