[ASTMatchers][NFC] Use SmallVector when building variadic matcher descriptor

[njames93]

Summary

Saves having to manually deallocate storage and keeps InnerArgs will have good cache locality.

[aaron.ballman]

LGTM!

clang/lib/ASTMatchers/Dynamic/Marshallers.h

 };
 
 /// Variadic marshaller function.
 template <typename ResultT, typename ArgT,
           ResultT (*Func)(ArrayRef<const ArgT *>)>
 VariantMatcher
 variadicMatcherDescriptor(StringRef MatcherName, SourceRange NameRange,
                           ArrayRef<ParserValue> Args, Diagnostics *Error) {
-  ArgT **InnerArgs = new ArgT *[Args.size()]();
+  SmallVector<ArgT *, 8> InnerArgsPtr;
+  InnerArgsPtr.resize_for_overwrite(Args.size());
+  SmallVector<ArgT, 8> InnerArgs;
+  InnerArgs.reserve(Args.size());
 
-  bool HasError = false;
   for (size_t i = 0, e = Args.size(); i != e; ++i) {
     using ArgTraits = ArgTypeTraits<ArgT>;
 
     const ParserValue &Arg = Args[i];
     const VariantValue &Value = Arg.Value;
     if (!ArgTraits::hasCorrectType(Value)) {
       Error->addError(Arg.Range, Error->ET_RegistryWrongArgType)
           << (i + 1) << ArgTraits::getKind().asString() << Value.getTypeAsString();
-      HasError = true;
-      break;
+      return {};
     }
     if (!ArgTraits::hasCorrectValue(Value)) {
       if (llvm::Optional<std::string> BestGuess =
               ArgTraits::getBestGuess(Value)) {
         Error->addError(Arg.Range, Error->ET_RegistryUnknownEnumWithReplace)
             << i + 1 << Value.getString() << *BestGuess;
       } else if (Value.isString()) {
         Error->addError(Arg.Range, Error->ET_RegistryValueNotFound)
             << Value.getString();
       } else {
         // This isn't ideal, but it's better than reporting an empty string as
         // the error in this case.
         Error->addError(Arg.Range, Error->ET_RegistryWrongArgType)
             << (i + 1) << ArgTraits::getKind().asString()
             << Value.getTypeAsString();
       }
-      HasError = true;
-      break;
-    }
-
-    InnerArgs[i] = new ArgT(ArgTraits::get(Value));
-  }
-
-  VariantMatcher Out;
-  if (!HasError) {
-    Out = outvalueToVariantMatcher(Func(llvm::makeArrayRef(InnerArgs,
-                                                           Args.size())));
+      return {};
     }
-
-  for (size_t i = 0, e = Args.size(); i != e; ++i) {
-    delete InnerArgs[i];
+    InnerArgs.set_size(i + 1);
+    InnerArgsPtr[i] = new (&InnerArgs[i]) ArgT(ArgTraits::get(Value));
   }
-  delete[] InnerArgs;
-  return Out;
+  return outvalueToVariantMatcher(Func(InnerArgsPtr));
 }
 
 /// Matcher descriptor for variadic functions.
 ///
 /// This class simply wraps a VariadicFunction with the right signature to export
 /// it as a MatcherDescriptor.
 /// This allows us to have one implementation of the interface for as many free
 /// functions as we want, reducing the number of symbols and size of the