[llvm-dev] Legacy PM deprecation for optimization pipeline timeline

[Arthur Eubanks via llvm-dev]

Splitting this off from the other thread.

We should have a deprecation timeline that revolves around the major
releases. I'd say we announce the legacy PM for the optimization pipeline
to be deprecated the major release after the new PM has been the default.
Looks like the new PM switch made it into LLVM 12.

The major blocker right now is that the C API doesn't have hooks into the
new PM infrastructure. llvm/include/llvm-c/Transforms/PassManagerBuilder.h
should be fairly straightforward to port to the new PM, but the API to add
individual passes (e.g. llvm/include/llvm-c/Transforms/Scalar.h) needs to
distinguish between the different types of passes, e.g. module vs function
pass. The new PM has explicit pass nesting, so we'll need to make sure that
we add a function pass to a function pass manager. Or we could simplify
things and force each pass added via the C API to run in isolation (e.g.
two adjacent function passes would run completely separately rather than
being interleaved function-by-function), which doesn't match how pipelines
are constructed everywhere else, but it's already an adhoc API.

At some point after the deprecation announcement we should start cleaning
up tests for passes in the optimization pipeline to use `opt -passes=foo`
rather than `opt -foo`.
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