Add RawStringOstream for slightly simpler streaming to strings (#4817)

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>

toolchain/language_server/language_server.cpp

@@ -6,6 +6,7 @@
 
 #include "clang-tools-extra/clangd/LSPBinder.h"
 #include "clang-tools-extra/clangd/Transport.h"
+#include "common/raw_string_ostream.h"
 #include "toolchain/language_server/context.h"
 #include "toolchain/language_server/incoming_messages.h"
 #include "toolchain/language_server/outgoing_messages.h"
@@ -26,10 +27,9 @@ auto Run(std::FILE* input_stream, llvm::raw_ostream& output_stream)
   // Run the server loop.
   llvm::Error err = transport->loop(incoming);
   if (err) {
-    std::string str;
-    llvm::raw_string_ostream out(str);
+    RawStringOstream out;
     out << err;
-    return Error(str);
+    return Error(out.TakeStr());
   } else {
     return Success();
   }