[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:
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