[analyzer] Only allow CallEvents to be created by CallEventManager.

This ensures that it is valid to reference-count any CallEvents, and we
won't accidentally try to reclaim a CallEvent that lives on the stack.
It also hides an ugly switch statement for handling CallExprs!

There should be no functionality change here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160986 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed