This addresses a TODO from D41406. I re-used PostImplicitCall program point when calling the new callback, but it indeed causes problems: because implicit call has no associated statement, MallocChecker was unable to put visitor diagnostics over these nodes. Create a new class of ProgramPoint, namely PostAllocatorCall, which is a sub-class of StmtPoint, and use that when calling the callback.
This patch might cause slight conflicts with D41150.
Maybe = default is getting more canonical within LLVM? But that would not match the rest of the file, so I am fine with not touching this.