[flang][driver] Use `-O{0|1|2|3}` to define LLVM backend pass pipeline

Support for optimisation flags in LLVM Flang was originally added in
https://reviews.llvm.org/D128043. That patch focused on LLVM
middle-end/optimisation pipelines. With this patch, Flang will
additionally configure LLVM backend pass pipelines accordingly. This
behavior is consistent with Clang.

New hook is added to translate compiler optimisation flags (e.g. `-O3`)
into backend optimisation level: `getCGOptLevel`. Identical hooks are
available in Clang and LLVM. In other words, the meaning of these
optimisation flags remains consistent with other sub-projects that use
LLVM backends.

Differential Revision: https://reviews.llvm.org/D128050