Revert "Revert "Support: Use std::unique_ptr for SignpostEmitter::Impl, NFC""

This reverts commit 07807228, reapplying
022ccedd.

I figured out why this was failing in other environments: it's not a
problem with std::unique_ptr, but that SignpostEmitterImpl only has a
forward declaration. Adding an empty definition should do the trick.

Original commit message:

    Replace some manual memory management with std::unique_ptr.

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