Layer the memory manager between the JIT and the runtime Dyld.

Layer the memory manager between the JIT and the runtime Dyld.

The JITMemory manager references LLVM IR constructs directly, while the
runtime Dyld works at a lower level and can handle objects which may not
originate from LLVM IR. Introduce a new layer for the memory manager to
handle the interface between them. For the MCJIT, this layer will be almost
entirely simply a call-through w/ translation between the IR objects and
symbol names.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128851 91177308-0d34-0410-b5e6-96231b3b80d8