c++: Fix comparison of fn() and ns::fn() [PR90711]

c++: Fix comparison of fn() and ns::fn() [PR90711]

The resolution of CWG issue 1321 clarified that when deciding whether two
expressions involving template parameters are equivalent, two dependent
function calls where the function is named with an unqualified-id are
considered to be equivalent if the name is the same, even if unqualified
lookup finds different sets of functions.  We were wrongly treating
qualified-ids the same way, so that EXISTS and test::EXISTS were considered
to be equivalent even though they are looking up the name in different
scopes.  This also causes a mangling bug, but I don't think it's safe to fix
that for GCC 10; this patch just fixes the comparison.

gcc/cp/ChangeLog
2020-03-30  Jason Merrill  <jason@redhat.com>

	PR c++/90711
	* tree.c (cp_tree_equal) [CALL_EXPR]: Compare KOENIG_LOOKUP_P.
	(called_fns_equal): Check DECL_CONTEXT.