Convert a few more cases of direct intialization of unique_ptrs from MemoryBuffer::getMemBuffer to move initialization now that it returns by unique_ptr instead of raw pointer.

Cleanup/improvements following r216583.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216605 91177308-0d34-0410-b5e6-96231b3b80d8