[llvm-objdump] Keep the memory buffer from the dSYM alive when using -g -dsym

When using -g and -dsym, llvm-objdump opens the dsym file and keeps the
MachOObjectFile alive, while the memory buffer that the MachOObjectFile
was based on gets destroyed.

Differential Revision: https://reviews.llvm.org/D51365

llvm-svn: 341209