[analyzer] Treat std::{move,forward} as casts in ExprMutationAnalyzer.

Summary:
This is a follow up of D52008 and should make the analyzer being able to handle perfect forwardings in real world cases where forwardings are done through multiple layers of function calls with `std::forward`.

Fixes PR38891.

Reviewers: lebedev.ri, JonasToth, george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin, mikhail.ramalho, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D52120

llvm-svn: 342409