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>

explorer/file_test.cpp

@@ -3,9 +3,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include "absl/flags/flag.h"
+#include "common/raw_string_ostream.h"
 #include "explorer/main.h"
 #include "re2/re2.h"
-#include "testing/base/test_raw_ostream.h"
 #include "testing/file_test/file_test_base.h"
 
 ABSL_FLAG(bool, trace, false,
@@ -103,7 +103,7 @@ class ExplorerFileTest : public FileTestBase {
     return test_name().find("/trace/") != std::string::npos;
   }
 
-  TestRawOstream trace_stream_;
+  RawStringOstream trace_stream_;
   RE2 prelude_line_re_;
   RE2 timing_re_;
 };