The goal of this patch is to enable utilizing LLVM plugin passes and
remarks for GPU offload code at link time. Specifically, this patch
extends clang-linker-wrapper's `--offload-opt` (and consequently
`-mllvm`) to accept the various LLVM pass options that tools like opt
usually accept. Those options include `--passes`, `--load-pass-plugin`,
and various remarks options.
Unlike many other LLVM options that are inherited from linked code by
clang-linker-wrapper (e.g., `-pass-remarks` is already implemented in
`llvm/lib/IR/DiagnosticHandler.cpp`), these options are implemented
separately as needed by each tool (e.g., opt, llc). Fortunately, this
patch is able to handle most of the implementation by passing the option
values to `lto::Config`.
For testing plugin support, this patch uses the simple `Bye` plugin from
LLVM core, but that requires several small Clang test suite config
extensions.
