c++: UNBOUND_CLASS_TEMPLATE context substitution [PR119981]

c++: UNBOUND_CLASS_TEMPLATE context substitution [PR119981]

In r15-123 and r14-11434 we unconditionally set processing_template_decl
when substituting the context of an UNBOUND_CLASS_TEMPLATE, in order to
handle instantiation of the dependently scoped friend declaration

  template<int N>
  template<class T>
  friend class A<N>::B;

where the scope A<N> remains dependent after instantiation.  But this
turns out to misbehave for the UNBOUND_CLASS_TEMPLATE in the below
testcase representing

  g<[]{}>::template fn

since with the flag set substituting the args of test3 into the lambda
causes us to defer the substitution and yield a lambda that still looks
dependent, which in turn makes g<[]{}> still dependent and not suitable
for qualified name lookup.

This patch restricts setting processing_template_decl during
UNBOUND_CLASS_TEMPLATE substitution to the case where there are multiple
levels of introduced template parameters, as in the friend declaration.
(This means we need to substitute the template parameter list(s) first,
which makes sense since they lexically appear first.)

	PR c++/119981
	PR c++/119378

gcc/cp/ChangeLog:

	* pt.cc (tsubst) <case UNBOUND_CLASS_TEMPLATE>: Substitute
	into template parameter list first.  When substituting the
	context, only set processing_template_decl if there's more
	than one level of introduced template parameters.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ15.C: New test.

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