PR c++/91378 - ICE with noexcept and auto return type.

PR c++/91378 - ICE with noexcept and auto return type.

Here, since the call to g is not type-dependent, we call mark_used on it to
determine its return type.  This also wants to instantiate the
noexcept-expression.  But since nothing in maybe_instantiate_noexcept was
calling push_to_top_level, we substituted b.i with processing_template_decl
set, so we left it unresolved for later access checking.  As a result, the
type of C::g<int> remained instantiation-dependent, leading to an ICE in
type_dependent_expression_p on the assert that the type of a function
template with no dependent template arguments must be non-dependent.

	* pt.c (maybe_instantiate_noexcept): push_to_top_level.

From-SVN: r274143