[Orc] Refactor ObjectLinkingLayer::addObjectSet to defer loading objects until

they're needed.

Prior to this patch objects were loaded (via RuntimeDyld::loadObject) when they
were added to the ObjectLinkingLayer, but were not relocated and finalized until
a symbol address was requested. In the interim, another object could be loaded
and finalized with the same memory manager, causing relocation/finalization of
the first object to fail (as the first finalization call may have marked the
allocated memory for the first object read-only).

By deferring the loadObject call (and subsequent memory allocations) until an
object file is needed we can avoid prematurely finalizing memory.

llvm-svn: 258185