[CallSiteInfo][NFC] CallSiteInfo -> CallSiteInfo.ArgRegPairs (#86842)

CallSiteInfo is originally used only for argument - register pairs. Make
it struct, in which we can store additional data for call sites.

Also, the variables/methods used for CallSiteInfo are named for its
original use case, e.g., CallFwdRegsInfo. Refactor these for the
upcoming
use, e.g. addCallArgsForwardingRegs() -> addCallSiteInfo().

An upcoming patch will add type ids for indirect calls to propogate them
from
middle-end to the back-end. The type ids will be then used to emit the
call
graph section.

Original RFC:
https://lists.llvm.org/pipermail/llvm-dev/2021-June/151044.html
Updated RFC:
https://lists.llvm.org/pipermail/llvm-dev/2021-July/151739.html

Differential Revision: https://reviews.llvm.org/D107109?id=362888



Co-authored-by: Necip Fazil Yildiran <necip@google.com>