[Flang] Lower achar intrinsic

The intrinsic returns the character located at the position requested
in the ASCII sequence. The intrinsic is lowered to inline FIR code.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D122480



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