[NFC] Rename attribute methods that work with indexes

This is part one of a couple of patches to fully rename these methods.

I've made the mistake of assuming that these indexes are for parameters
multiple times, but actually they're based off of a weird indexing
scheme AttributeList::AttrIndex where 0 is the return value and ~0 is
the function. Hopefully renaming these methods will make this clearer.
Ideally users should use more specific methods like
AttributeList::getFnAttr().

This patch simply adds the name that we want in the end. This is so the
removal of the methods with the original names happens in a separate
change to make it easier for downstream users.

This touches all relevant methods in AttributeList, CallBase, and Function.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D108788