Default raw_string_ostream to be unbuffered

raw_string_ostream can just use the std::string as a buffer. The buffer
requirement came from the days when the minimum buffer size was 128
(fixed in 2015) and std::string was non-SSO. Now we can just use the
inline capacity for small things and on a good growth strategy later.

This assumes that the standard library isn't doing something bad like
only growing to the exact size. I checked some common implementations
and they grow by 2x (libc++) or 1.5x (msvc) which is reasonable. We
should still check if this incurs any performance regressions though.

llvm/include/llvm/Support/raw_ostream.h

@@ -524,7 +524,9 @@ class raw_string_ostream : public raw_ostream {
   uint64_t current_pos() const override { return OS.size(); }
 
 public:
-  explicit raw_string_ostream(std::string &O) : OS(O) {}
+  explicit raw_string_ostream(std::string &O) : OS(O) {
+    SetUnbuffered();
+  }
   ~raw_string_ostream() override;
 
   /// Flushes the stream contents to the target string and returns  the string's

llvm/unittests/Support/raw_ostream_test.cpp

@@ -18,7 +18,10 @@ namespace {
 
 template<typename T> std::string printToString(const T &Value) {
   std::string res;
-  return (llvm::raw_string_ostream(res) << Value).str();
+  llvm::raw_string_ostream OS(res);
+  OS.SetBuffered();
+  OS << Value;
+  return res;
 }
 
 /// printToString - Print the given value to a stream which only has \arg