[analyzer][NFC] Merge checkNewAllocator's paramaters into CXXAllocatorCall

Party based on this thread:
http://lists.llvm.org/pipermail/cfe-dev/2020-February/064754.html.

This patch merges two of CXXAllocatorCall's parameters, so that we are able to
supply a CallEvent object to check::NewAllocatorCall (see the description of
D75430 to see why this would be great).

One of the things mentioned by @NoQ was the following:

  I think at this point we might actually do a good job sorting out this
  check::NewAllocator issue because we have a "separate" "Environment" to hold
  the other SVal, which is "objects under construction"! - so we should probably
  simply teach CXXAllocatorCall to extract the value from the
  objects-under-construction trait of the program state and we're good.

I had MallocChecker in my crosshair for now, so I admittedly threw together
something as a proof of concept. Now that I know that this effort is worth
pursuing though, I'll happily look for a solution better then demonstrated in
this patch.

Differential Revision: https://reviews.llvm.org/D75431