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/base/error_builders_test.cpp

@@ -6,8 +6,8 @@
 
 #include <gtest/gtest.h>
 
+#include "common/raw_string_ostream.h"
 #include "explorer/base/source_location.h"
-#include "testing/base/test_raw_ostream.h"
 
 namespace Carbon {
 namespace {
@@ -17,7 +17,7 @@ TEST(ErrorBuildersTest, ProgramError) {
   EXPECT_EQ(err.location(), "x:1");
   EXPECT_EQ(err.message(), "test");
 
-  Testing::TestRawOstream out;
+  RawStringOstream out;
   out << err;
   EXPECT_EQ(out.TakeStr(), "x:1: test");
 }