c++: Instantiation with local extern [PR97395]

c++: Instantiation with local extern [PR97395]

It turns out that pushdecl_with_scope has somewhat strange behaviour,
which probably made more sense way back.  Unfortunately making it
somewhat saner turned into a rathole.  Instead use a
push_nested_namespace around pushing the alias -- this is similar to
some of the friend handling we already have.

	gcc/cp/
	* name-lookup.c (push_local_extern_decl_alias): Push into alias's
	namespace and use pushdecl.
	(do_pushdecl_with_scope): Clarify behaviour.
	gcc/testsuite/
	* g++.dg/lookup/extern-redecl2.C: New.