[Orc] Use extensible RTTI for the orc::ObjectLayer class hierarchy

So far we had no way to distinguish between JITLink and RuntimeDyld in lli. Instead, we used implicit knowledge that RuntimeDyld would be used for linking ELF. In order to get D97337 to work with lli though, we have to move on and allow JITLink for ELF. This patch uses extensible RTTI to allow external clients to add their own layers without touching the LLVM sources.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D97338