Convert fir.allocmem and fir.freemem operations to calls to malloc and free, respectively

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

Address review comments
- move CHECK blocks to after the mlir code in the test file
- fix style with respect to anonymous namespaces: only include class definitions in the namespace and make functions static and outside the namespace
- fix a few nits
- remove TODO in favor of notifyMatchFailure
- removed unnecessary CHECK line from convert-to-llvm.fir
- rebase on main - add TODO back in
- get successfull test of TODO in AllocMemOp converion of derived type with LEN params
- clearer comments and reduced use of auto
- move defintion of computeDerivedTypeSize to fix build error

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>

Reviewed By: awarzynski, clementval, kiranchandramohan, schweitz

Differential Revision: https://reviews.llvm.org/D114104