[flang] Switch return to ExtendedValue in AbstractConverter and Bridge

Change the signature of `genExprAddr`, `genExprValue` to return a `fir::ExtendedValue` instead of a simple `mlir::Value`

This patch is a preparation for more lowering to be upstream. It supports D118786 and D118787.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118785