[AutoDiff] NFC: Reimplement PullbackCloner using pimpl pattern. (swiftlang#32778)

Reimplement `PullbackCloner` using the pointer-to-implementation pattern.

`PullbackCloner.h` is now tiny: `PullbackCloner` exposes only a `bool run()`
entry point.

All of the implementation is moved to `PullbackCloner::Implementation` in
`PullbackCloner.cpp`.

Benefits of this approach:

- A main benefit is that methods can be defined directly in `PullbackCloner.cpp`
  without needing to separately declare them in `PullbackCloner.h`.
  - There is now no code duplication between `PullbackCloner.h` and
    `PullbackCloner.cpp`.
  - Consequently, method documentation is easier to read because it appears
    directly on method definitions, instead of on method declarations in a
    separate file. This is important for documentation of `PullbackCloner`
    instruction visitor methods, which explain pullback transformation rules.
- Incremental recompilation may be faster since `PullbackCloner.h` changes less
  often.

Partially resolves SR-13182.

Author: dan-zheng