Support: Use std::unique_ptr for SignpostEmitter::Impl, NFC, 3rd attempt

This reverts commit e35afbe5, reapplying
022ccedd and
e7ed5c92.

- The first attempt missed defining `SignpostEmitterImpl`.
- The second attempt missed defining `llvm::SignpostEmitterImpl`.

Not sure how I failed to test both versions locally before; I thought
I'd turned the feature off via rerunning `cmake` but it must have been
stuck in place. This time I confirmed via `clang -E` that I was testing
both build configurations.

Original commit message:

    Replace some manual memory management with std::unique_ptr.

    Differential Revision: https://reviews.llvm.org/D100151