abstract information about the callee.  NFC.

The goal here is to make it easier to recognize indirect calls and
trigger additional logic in certain cases.  That logic will come in
a later patch; in the meantime, I felt that this was a significant
improvement to the code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285258 91177308-0d34-0410-b5e6-96231b3b80d8