[patch][rfc] Add pwrite to raw_stream

[Rafael Espíndola]

I noticed that object writer are somewhat convoluted because they have
to write to streams with to seek or pwrite support. For that to work
they have to do a pass over the data computing the size and only then
write it out.

That is a bit silly, as most of the time (and the ones we care about
performance) we are writing to an actual file with seek support.

When that is not the case, we can accumulate the data and write it on the end.

For accumulating the data, raw_svector_ostream works just fine, except
that it doesn't support seeking. Adding that is doable, but
complicates a performance critical structure.

Fortunately, adding pwrite to raw_svector_ostream is trivial.

The attached patch changes ELFObjectWriter.cpp to use pwrite to update
the one forward reference that is left at that point. Similar cleanups
should be possible for the other object writers.

The patch also adds a virtual pwrite to raw_stream and a helper class
for creating a raw_svector_ostream when a given FD does'n support
seeking (like a pipe).

It passes check-llvm with the exception of
CodeGen/R600/cf-stack-bug.ll which uses "2>&1" and depends on the
order the data from stdout and stderr is mixed.

The test could be fixed, but another issue is that clang is structured
to pass a plain raw_ostream around until the last moment when it is
wrapped in a formatted_raw_ostream. That makes it harder to detect if
a stream supports seeking or not.

So, if you guys are OK with this direction, I think the proper way to
split/commit this is

* Implement classof for raw_ostream.
* Stop using formatted_raw_ostream when producing binaries.
* This patch, but SeekWrapper can then take a plain raw_stream and use
dyn_cast to see if it a fd stream. It would also not add pwrite to
formatted_raw_ostream.

Cheers,
Rafael
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