Make decl_base::get_qualified_name() work when decl context changes

Make decl_base::get_qualified_name() work when decl context changes

decl_base::get_qualified_name() caches its result.  So when it's
first called on a decl that is not added to a scope, what is returned
is a non-qualified name.  Which is all right.  But then when the decl
is later added to a scope, the cached result of
decl_base::get_qualified_name() is not longer correct.

This patch resets the cache of decl_base::get_qualified_name() when
the decl gets added to a new scope.

	* include/abg-ir.h (class decl_base): Make class scope_decl a
	friend of decl_base.
	(type_base::priv_): Make this protected, rather than private.
	* src/abg-ir.cc (scope_decl::add_member_decl)
	(scope_decl::insert_member_decl): Reset the cache of the result of
	decl_base::get_qualified_name().
	* tests/data/test-abidiff/test-PR18791-report0.txt: Adjust.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>