Support: Skip buffering buffer_unique_ostream's owned stream

Change buffer_unique_ostream's constructor to call
raw_ostream::SetUnbuffered() on its owned stream. Otherwise,
buffer_unique_ostream's destructor could cause the owned stream to
temporarily allocate a buffer only to be immediately flushed.

Also add some tests for buffer_ostream and buffer_unique_ostream. Use
the same naming scheme as other raw_ostream-related tests (e.g.,
`raw_ostreamTest` for the fixture, `raw_ostream_test.cpp` for the
filename).

(I considered changing buffer_ostream in the same way (calling
SetUnbuffered on the referenced stream), but that seemed like overreach
since the client may have more things to write.)

(I considered merging buffer_ostream and buffer_unique_ostream into a
single class (with a `raw_ostream&` and a `std::unique_ptr` that is only
sometimes used), but that makes the class bigger and the small amount of
code deduplication seems uncompelling.)

Differential Revision: https://reviews.llvm.org/D110369