[fir] Add test for FIR types conversion

Add a separate file to test FIR types conversion to LLVM types.
Conversion comes from `flang/lib/Optimizer/CodeGen/TypeConverter.h`

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

Reviewed By: kiranchandramohan, awarzynski

Differential Revision: https://reviews.llvm.org/D113283