[llvm] r208021 - [Support/MemoryBuffer] Move the IsVolatile check inside shouldUseMmap() and make sure to zero-initialize the rest

[Rafael Espíndola]

> +      assert(IsVolatile && "We got inaccurate FileSize value or fstat reported "
> +                           "an invalid file size.");
> +      memset(BufPtr, 0, BytesLeft); // zero-initialize rest of the buffer.

This can be reached because of uses error, right? Editing a file at
the right moment while clang is running for example. If that is the
case, this should probably be a report_fatal_error instead of an
assert.

Cheers,
Rafael