NFC: Move ObjectMemoryBuffer to support

Summary:
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

llvm-svn: 330093