MemoryBuffer: remove unusued definitions

These were made redundant back in r186560.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208395 91177308-0d34-0410-b5e6-96231b3b80d8
MemoryBuffer: don't force mmap when stat fails

Fix error handling introduced in r127426 that could result in MemoryBuffers not
having null termination.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208396 91177308-0d34-0410-b5e6-96231b3b80d8