Frontend: Simplify handling of non-seeking streams in CompilerInstance, NFC

Add a new `raw_pwrite_ostream` variant, `buffer_unique_ostream`, which
is like `buffer_ostream` but with unique ownership of the stream it's
wrapping. Use this in CompilerInstance to simplify the ownership of
non-seeking output streams, avoiding logic sprawled around to deal with
them specially.

This also simplifies future work to encapsulate output files in a
different class.

Differential Revision: https://reviews.llvm.org/D93260