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/driver/format_subcommand.cpp

@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include "common/raw_string_ostream.h"
 #include "toolchain/base/shared_value_stores.h"
 #include "toolchain/diagnostics/diagnostic_consumer.h"
 #include "toolchain/format/format.h"
@@ -83,15 +84,14 @@ auto FormatSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
     SharedValueStores value_stores;
     auto tokens = Lex::Lex(value_stores, *source, consumer);
 
-    std::string buffer_str;
-    llvm::raw_string_ostream buffer(buffer_str);
-
+    RawStringOstream buffer;
     if (Format::Format(tokens, buffer)) {
       // TODO: Figure out a multi-file output setup that supports good
       // multi-file testing.
       // TODO: Use --output values (and default to overwrite).
-      driver_env.output_stream << buffer_str;
+      driver_env.output_stream << buffer.TakeStr();
     } else {
+      buffer.clear();
       mark_per_file_error();
       driver_env.output_stream << source->text();
     }