Enable the use of pwrite in the object writers.

[Duncan P. N. Exon Smith]

> On 2015-Apr-10, at 14:35, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
>> * Have buffer_ostream contain a buffer. That is probably the best for
>> the users, but requires changing raw_svector_ostream to have a
>> protected init method since it would be constructed before the buffer.
>> I might still give this one a try.
> 
> That was fairly easy. So patches with it are attached. I like it a bit better
> 
> Another possible variation that I forgot to mention on the original email:
> 
> Have LLVMTargetMachine::addPassesToEmitFile create the buffer stream
> when seeking is not supported. The disadvantage is that it would
> require a "supportsSeeking()" method in the base, so it looks like a
> clumsy version of isa/dyn_cast. It would also not solve clang's other
> uses of streams.

Both this and the previous mail have callers of `supportsSeeking()` but no
definition (in llvm.patch).  Is the patch out-of-sync somehow?

> 
> Cheers,
> Rafael
> <llvm.patch><clang.patch>