Correct invalid debug info metadata

Code review feedback on r176838 by Patrik Hägglund.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176884 91177308-0d34-0410-b5e6-96231b3b80d8
[Support][MemoryBuffer] Use sys::fs::mapped_file_region instead of sys::Path::MapInFilePages.

This gives us memory mapped file I/O on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176886 91177308-0d34-0410-b5e6-96231b3b80d8