[Support] Make raw_string_ostream unbuffered

Summary:
In D58580 i have noted that `llvm::to_string()` is a memory hog.
It uses `raw_string_ostream`, and since it was buffered,
every `raw_string_ostream` had a cost of `BUFSIZ` bytes
(which is `8192` at least here). So every `llvm::to_string()`
call, even to just print an `int`, costed `8192` bytes.

In D58580, getting rid of that buffering //had// significant
performance and memory consumption improvements for `llvm-xray convert`.

Similarly, in D58580 @rnk pointed out that the `raw_svector_ostream`
is already unbuffered, and `write_unsigned_impl` and friends
do internal buffering. So it should be ok performance-wise to just
make the `raw_string_ostream` itself unbuffered.

Here, i don't have any perf measurements.
Another letdown is that i'm leaving a loose-end - not deleting the
`flush()` method. I  don't expect that cleanup to be anything more
than just fixing every new compiler error, but i'm presently unable
to do that. Will look into that later.

Reviewers: rnk, zturner

Reviewed By: rnk

Subscribers: kristina, jdoerfert, llvm-commits, rnk

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58643

llvm-svn: 354819