c++: ICE with -Wshadow and enumerator in template [PR99120]

c++: ICE with -Wshadow and enumerator in template [PR99120]

We crash here, because in a template, an enumerator doesn't have
a type until we've called finish_enum_value_list.  But our -Wshadow
implementation, check_local_shadow, is called when we pushdecl in
build_enumerator, which takes place before finish_enum_value_list.

gcc/cp/ChangeLog:

	PR c++/99120
	* name-lookup.c (check_local_shadow): Check if the type of decl
	is non-null before checking TYPE_PTR*.

gcc/testsuite/ChangeLog:

	PR c++/99120
	* g++.dg/warn/Wshadow-17.C: New test.