## Problem
Given 3 pipelines, A, B, and a superset pipeline AB that runs both the A
& B pipelines, it is not easy to manage their options - one needs to
manually recreate all options from A and B into AB, and maintain them.
This is tedious.
## Proposed solution
Ideally, AB options class inherits from both A and B options, making the
maintenance effortless. Today though, this causes problems as their base
classes `PassPipelineOptions<A>` and `PassPipelineOptions<B>` both
inherit from `mlir::detail::PassOptions`, leading to the so called
"diamond inheritance problem", i.e. multiple definitions of the same
symbol, in this case parseFromString that is defined in
mlir::detail::PassOptions.
Visually, the inheritance looks like this:
```
mlir::detail::PassOptions
↑ ↑
| |
PassPipelineOptions<A> PassPipelineOptions<B>
↑ ↑
| |
AOptions BOptions
↑ ↑
+---------+--------+
|
ABOptions
```
A proposed fix is to use the common solution to the diamond inheritance
problem - virtual inheritance.
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