Don't allow pwrite to resize a stream.

The current implementations could exhibit some behavior differences:

raw_fd_ostream: Whatever the underlying fd does with seek+write. In a normal
file, the write position would be back to the old offset.

raw_svector_ostream: The write position is always the end of the stream, so
after pwrite the write position would be the new end. This matches what OS_X
(all BSD?) do with a pwrite in a O_APPEND fd.

Given that we don't need that feature and don't use O_APPEND a lot in LLVM,
just disallow it.

I am open to suggestions on renaming pwrite to something else, but this fixes
the issue for now.

Thanks to Yaron Keren for reporting it.

llvm-svn: 235303