[IR] Add additional addParamAttr/removeParamAttr to AttributeList API

Summary:
Fairly straightforward patch to fill in some of the holes in the
attributes API with respect to accessing parameter/argument attributes.
The patch aims to step further towards encapsulating the
idx+FirstArgIndex pattern to access these attributes to within the
AttributeList.

Patch by Daniel Neilson!

Reviewers: rnk, chandlerc, pete, javed.absar, reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33355

llvm-svn: 304329