Canonicalize declaration pointers when forming APValues.

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 e6393ee813178e9d3306b8e3c6949a4f32f8a2cb with fixed
handling for weak declarations. We now look for attributes on the most
recent declaration when determining whether a declaration is weak.