This adds a RawStringOstream. Versus TestRawOstream, which is
consolidated over to RawStringOstream, it uses a string for storage
instead of a vector, mainly to support move-to-string semantics. Versus
llvm::raw_string_ostream, it owns the string and supports pwrite (which
is needed for driver and its fd_ostream compatibility requirement).
This converts most uses of llvm::raw_string_ostream, leaving behind a
few in InstNamer that explicitly cannot own the string, such as:
```
llvm::raw_string_ostream(name)
<< "_" << tree.tokens().GetColumnNumber(token);
```
I have this as its own library so that it can use CHECK.
Yes this doesn't save much code, but it's code we repeatedly write.
---------
Co-authored-by: Geoff Romer <gromer@google.com>
