Treat associated types the same as type parameters when it comes to region bounding...

Treat associated types the same as type parameters when it comes to region bounding. Fixes #20303.

Strictly speaking, this is a [breaking-change] (if you are using
associated types). You are no longer free to wantonly violate the type
system rules by closing associated types into objects without any form
of region bound. Instead you should add region bounds like `T::X :
'a`, just as you would with a normal type parameter.