[analyzer] operator new: Add a new ProgramPoint for check::NewAllocator.

Add PostAllocatorCall program point to represent the moment in the analysis
between the operator new() call and the constructor call. Pointer cast from
"void *" to the correct object pointer type has already happened by this point.

The new program point, unlike the previously used PostImplicitCall, contains a
reference to the new-expression, which allows adding path diagnostics over it.

Differential Revision: https://reviews.llvm.org/D41800
rdar://problem/12180598

llvm-svn: 322796