[CallSite removal] Add and flesh out APIs on the new `CallBase` base class...

[CallSite removal] Add and flesh out APIs on the new `CallBase` base class that previously were only available on the `CallSite` wrapper.

Summary:
This will make migrating code easier and generally seems like a good collection
of API improvements.

Some of these APIs seem like more consistent / better naming of existing
ones. I've retained the old names for migration simplicit and am just
adding the new ones in this commit. I'll try to garbage collect these
once CallSite is gone.

Subscribers: sanjoy, mcrosier, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D55638

llvm-svn: 350109