[fir] Add base of the FIR to LLVM IR pass

This patch adds the base of the FIR to LLVM IR Dialect conversion pass.
It currently can convert the following operations:
 - fir.global
 - fir.has_value
 - fir.address_of
 - fir.undefined

This patch is part of the upstreaming effort from fir-dev branch. It does not
cover all FIR operations in order to have small patches. Several patches will
follow to convert other operations.

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D112845