[Support] Add raw_ostream_iterator: ostream_iterator for raw_ostream
Adds a class `raw_ostream_iterator` that behaves like
std::ostream_iterator, but can be used with raw_ostream.
This is useful for using raw_ostream with std algorithms.
For example, it can be used to output std containers as follows:
```
std::vector<int> V = { 1, 2, 3 };
std::copy(V.begin(), V.end(), raw_ostream_iterator<int>(outs(), ", "));
// Output: "1, 2, 3, "
```
The API tries to follow std::ostream_iterator as closely as is
practically possible.
Reviewed By: dblaikie, mkitzan
Differential Revision: https://reviews.llvm.org/D78795