[CallSite removal] Move the rest of IR implementation code away from
`CallSite`.

With this change, the remaining `CallSite` usages are just for
implementing the wrapper type itself.

This does update the C API but leaves the names of that API alone and
only updates their implementation.

Differential Revision: https://reviews.llvm.org/D56184

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350509 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed