Add missing vtable anchors

Summary: This patch adds anchor() for MemoryBuffer, raw_fd_ostream, RTDyldMemoryManager, SectionMemoryManager, etc.

Reviewers: jlebar, eli.friedman, dblaikie

Reviewed By: dblaikie

Subscribers: mehdi_amini, mgorny, dblaikie, weimingz, llvm-commits

Differential Revision: https://reviews.llvm.org/D45244

llvm-svn: 329861