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>

common/vlog_test.cpp

@@ -7,7 +7,7 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
-#include "testing/base/test_raw_ostream.h"
+#include "common/raw_string_ostream.h"
 
 namespace Carbon::Testing {
 namespace {
@@ -30,7 +30,7 @@ class VLogger {
   auto TakeStr() -> std::string { return buffer_.TakeStr(); }
 
  private:
-  TestRawOstream buffer_;
+  RawStringOstream buffer_;
 
   llvm::raw_ostream* vlog_stream_ = nullptr;
 };
@@ -50,7 +50,7 @@ TEST(VLogTest, Disabled) {
 }
 
 TEST(VLogTest, To) {
-  TestRawOstream buffer;
+  RawStringOstream buffer;
   CARBON_VLOG_TO(&buffer, "Test");
   EXPECT_THAT(buffer.TakeStr(), "Test");
 }