NFC: Move ObjectMemoryBuffer to support
Since the class is used by both MCJIT and LTO, it makes more sense to move it to Support lib.
This is a follow up patch to r329929 and https://reviews.llvm.org/D45244
Reviewers: bkramer, dblaikie
Reviewed By: bkramer
Subscribers: mehdi_amini, eraman, llvm-commits
Differential Revision: https://reviews.llvm.org/D45606