ASTMatchers: Avoid using SmallVector::set_size()

[dexonsmith]

Summary

Update variadicMatcherDescriptor to assert on reserved capacity and
to call emplace_back() instead of calling set_size() and constructing
the element in-place.

[dblaikie]

Looks good!

clang/lib/ASTMatchers/Dynamic/Marshallers.h

if (!ArgTraits::hasCorrectValue(Value)) {
         // 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();
       }
       return {};
     }
-    InnerArgs.set_size(i + 1);
-    InnerArgsPtr[i] = new (&InnerArgs[i]) ArgT(ArgTraits::get(Value));
+    assert(InnerArgs.size() < InnerArgs.capacity());
+    InnerArgs.emplace_back(ArgTraits::get(Value));
+    InnerArgsPtr[i] = &InnerArgs[i];
   }
   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.