[Orc] Make the ObjectLinkingLayer take ownership of object files until

[Orc] Make the ObjectLinkingLayer take ownership of object files until
finalization time.

As currently implemented, RuntimeDyldELF requires the original object
file to be avaible when relocations are being resolved. This patch
ensures that the ObjectLinkingLayer preserves it until then. In the
future RuntimeDyldELF should be rewritten to remove this requirement, at
which point this patch can be reverted.

Regression test cases for Orc (which include coverage of this bug) will
be committed shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227778 91177308-0d34-0410-b5e6-96231b3b80d8