c++: block-scope extern decl with default args

c++: block-scope extern decl with default args

In adding the DECL_LOCAL_DECL handling, I'd forgotten that the
parm-decls also need cloning -- and resetting of their DECL_CONTEXT.
Also, any default args need droping when adding an alias, as those are
not propagated.  The std's not totally clear on this latter point when
there's no exising namespace decl, but that seems like the right thing
and is what clang does.

	gcc/cp/
	* name-lookup.c (push_local_extern_decl_alias): Reconstextualize
	alias' parm decls.  Drop any default args.
	gcc/testsuite/
	* g++.dg/lookup/local-extern.C: New.