Make ObjectFile ownership of the MemoryBuffer optional.

This allows llvm-ar to mmap the input files only once.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200040 91177308-0d34-0410-b5e6-96231b3b80d8
Fix c++03 build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200042 91177308-0d34-0410-b5e6-96231b3b80d8