Improve C++ CFG analysis about fuzz-introspector HOT 2 OPEN

Handling of VTable calls is now supported to an extend by Inspector::extractVTableIndirectCall

However, there is a limitation here which is we may not get the exact right class when extracting the target class, but rather a parent class. For example, the callgraph for: https://github.com/ossf/fuzz-introspector/blob/main/examples/cpp-simple-example-1/fuzzer.cpp

looks as follows:

0 LLVMFuzzerTestOneInput
  1 operator new(unsigned long)
  1 C::C()
    2 B::B() 
  1 B::bar() 

whereas the true callgraph is:

0 LLVMFuzzerTestOneInput
  1 operator new(unsigned long) 
  1 C::C() 
    2 B::B() 
  1 C::bar()

from fuzz-introspector.

This is not a trivial problem to solve at the IR level.

One way of potentially identifying this is backtracking on the LLVM IR to identify the constructor used to create a given object. We should be able to do this by looking at calls to new.

from fuzz-introspector.