[ORC] Make RuntimeDyldObjectLinkingLayer2 take memory managers by unique_ptr.

The existing memory manager API can not be shared between objects when linking
concurrently (since there is no way to know which concurrent allocations were
performed on behalf of which object, and hence which allocations would be safe
to finalize when finalizeMemory is called). For now, we can work around this by
requiring a new memory manager for each object.

This change only affects the concurrent version of the ORC APIs.

llvm-svn: 341579