[Support/MemoryBuffer] Move the IsVolatile check inside shouldUseMmap() and make sure to zero-initialize the rest
of the buffer if we unexpectedly reach end-of-file while reading.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208021 91177308-0d34-0410-b5e6-96231b3b80d8