Canonicalize declaration pointers when forming APValues.

References to different declarations of the same entity aren't different
values, so shouldn't have different representations.

Recommit of e6393ee8 with fixed handling
for weak declarations. We now look for attributes on the most recent
declaration when determining whether a declaration is weak. (Second
recommit with further fixes for mishandling of weak declarations. Our
behavior here is fundamentally unsound -- see PR47663 -- but this
approach attempts to not make things worse.)