[mlir] Fix -Wdangling-assignment-gsl in OperationSupport.h

This warning is causing lots of build spam when I use a recent Clang as
my host compiler. a3b4d91 fixes the
same issue in clangd; apply that fix here too. I also think this is a
potential false positive though. Fix variable casing while I'm here.

Author: smeenai

mlir/include/mlir/IR/OperationSupport.h

@@ -997,13 +997,15 @@ struct OperationState {
     if (!properties) {
       T *p = new T{};
       properties = p;
-      propertiesDeleter = [](OpaqueProperties prop) {
+      constexpr auto deleter = [](OpaqueProperties prop) {
         delete prop.as<const T *>();
       };
-      propertiesSetter = [](OpaqueProperties new_prop,
-                            const OpaqueProperties prop) {
-        *new_prop.as<T *>() = *prop.as<const T *>();
+      propertiesDeleter = deleter;
+      constexpr auto setter = [](OpaqueProperties newProp,
+                                 const OpaqueProperties prop) {
+        *newProp.as<T *>() = *prop.as<const T *>();
       };
+      propertiesSetter = setter;
       propertiesId = TypeID::get<T>();
     }
     assert(propertiesId == TypeID::get<T>() && "Inconsistent properties");