[CallSite removal] Add `CallBase` support to the `InstVisitor` in such

a way that it still supports `CallSite` but users can be ported to rely
on `CallBase` instead.

This will unblock the ports across the analysis and transforms libraries
(and out-of-tree users) and once done we can clean this up by removing
the `CallSite` layer.

Differential Revision: https://reviews.llvm.org/D56182

llvm-svn: 350502