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/driver_test.cpp

@@ -11,17 +11,16 @@
 #include <fstream>
 #include <utility>
 
+#include "common/raw_string_ostream.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Object/Binary.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "testing/base/global_exe_path.h"
-#include "testing/base/test_raw_ostream.h"
 #include "toolchain/testing/yaml_test_helpers.h"
 
 namespace Carbon {
 namespace {
 
-using ::Carbon::Testing::TestRawOstream;
 using ::testing::_;
 using ::testing::ContainsRegex;
 using ::testing::HasSubstr;
@@ -94,8 +93,8 @@ class DriverTest : public testing::Test {
   llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> fs_ =
       new llvm::vfs::InMemoryFileSystem;
   const InstallPaths installation_;
-  TestRawOstream test_output_stream_;
-  TestRawOstream test_error_stream_;
+  RawStringOstream test_output_stream_;
+  RawStringOstream test_error_stream_;
 
   // Some tests work directly with files in the test temporary directory.
   std::filesystem::path test_tmpdir_;