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

[Rafael Espíndola]

>> 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.
>
> Are you saying if IsVolatile is false it should crash the compiler ?
> I'm not fond of crashing the compiler because of user actions that we know how to recover from.

The issue right now is that we behave differently with and without asserts:

* With asserts we crash.
* Without asserts we zero part of the buffer.

Asserts should be reserved for things that only fire because we (llvm
and clang developers) made a mistake. That is, in a bug. Maybe the
correct solution is to just delete the assert? I think the net result
would be:

* IsVolatileSize forces us to use read instead of mmap.
* If for any reason we use read, we can cope with the file size
changing. The buffer itself will contain some mix of old and new data,
but the null terminator will be there.
* If using mmap we will have to report fatal errors since we cannot
guarantee that the null terminator will be there if the file size
changes.

Cheers,
Rafael