[NFC][YAMLTraits] Use SmallString<>+SSO when yamlizing non-strings

This gets rid of basically all temporary allocations
during YAML serialization of llvm-exegesis snippets,
we go from ~1.8M temporary allocations to 13k.

Author: LebedevRI

llvm/include/llvm/Support/YAMLTraits.h

@@ -1000,8 +1000,8 @@ template <typename T>
 std::enable_if_t<has_ScalarTraits<T>::value, void> yamlize(IO &io, T &Val, bool,
                                                            EmptyContext &Ctx) {
   if ( io.outputting() ) {
-    std::string Storage;
-    raw_string_ostream Buffer(Storage);
+    SmallString<128> Storage;
+    raw_svector_ostream Buffer(Storage);
     ScalarTraits<T>::output(Val, io.getContext(), Buffer);
     StringRef Str = Buffer.str();
     io.scalarString(Str, ScalarTraits<T>::mustQuote(Str));