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/testing/yaml_test_helpers.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/testing/yaml_test_helpers.h"
 
+#include "common/raw_string_ostream.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/YAMLParser.h"
 
@@ -63,9 +64,9 @@ auto Value::FromText(llvm::StringRef text) -> ErrorOr<SequenceValue> {
   sm.setDiagHandler(
       [](const llvm::SMDiagnostic& diag, void* context) -> void {
         auto* error_message = static_cast<std::optional<std::string>*>(context);
-        *error_message = std::string();
-        llvm::raw_string_ostream stream(**error_message);
+        RawStringOstream stream;
         diag.print(/*ProgName=*/nullptr, stream, /*ShowColors=*/false);
+        *error_message = stream.TakeStr();
       },
       &error_message);
   llvm::yaml::Stream yaml_stream(text, sm);