[analyzer] Deadstore static analysis: Fix false positive on C++17 assignments

Dead store detection automatically checks that an expression is a
CXXConstructor and skips it because of potential side effects. In C++17,
with guaranteed copy elision, this check can fail because we actually
receive the implicit cast of a CXXConstructor.
Most checks in the dead store analysis were already stripping all casts
and parenthesis and those that weren't were either forgotten (like the
constructor) or would not suffer from it, so this patch proposes to
factorize the stripping.
It has an impact on where the dead store warning is reported in the case
of an explicit cast, from

  auto a = static_cast<B>(A());
           ^~~~~~~~~~~~~~~~~~~

to

  auto a = static_cast<B>(A());
                          ^~~

which we think is an improvement.

Patch By: frederic-tingaud-sonarsource

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D126534