Add DeclAttribute::isEquivalent()

Adds a new method to `DeclAttribute` which can compare two attributes and see if they would be “equivalent” in light of the given decl. “Equivalent” here means that they would have the same effect on the declaration; for instance, two attrs with different source locations can be equivalent, and two attributes with the same arguments in a different order are equivalent if the ordering of those argumetns is not semantically equivalent.

This capability is not yet used in this commit, but `@abi` will check if certain attributes are equivalent or not.

Author: beccadax