[JITLink][Orc] Add MemoryMapper interface with InProcess implementation

MemoryMapper class takes care of cross-process and in-process address space
reservation, mapping, transferring content and applying protections.

Implementations of this class can support different ways to do this such
as using shared memory, transferring memory contents over EPC or just
mapping memory in the same process (InProcessMemoryMapper).

Reviewed By: sgraenitz, lhames

Differential Revision: https://reviews.llvm.org/D127491