[CodeGen] Add public function to emit C++ destructor call.

Adds `CodeGen::getCXXDestructorImplicitParam`, to retrieve a C++ destructor's implicit parameter (after the "this" pointer) based on the ABI in the given CodeGenModule.

This will allow other frontends (Swift, for example) to easily emit calls to object destructors with correct ABI semantics and calling convetions.

This is needed for Swift C++ interop. Here's the corresponding Swift change: https://github.com/apple/swift/pull/32291

Differential Revision: https://reviews.llvm.org/D82392