[flang] Basic local variable lowering

This patch add lowering for simple local variable.

- The signatures in `ConvertType.h` have been simplified to take advantage of the `AbstractConverter`.
- The lowering make use of the `allocateLocal` from the `FirOpBuilder`.

This lowering is used in patch D118982

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

Reviewed By: kiranchandramohan, jeanPerier, schweitz

Differential Revision: https://reviews.llvm.org/D118978