[IRReader] Expose getLazyIRModule

Currently there is no way to lazy-load an in-memory IR module without
first writing it to disk. This patch just exposes the existing
implementation of getLazyIRModule.

This is effectively a revert of rL212364

Differential Revision: https://reviews.llvm.org/D56203

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353755 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed