Reland "[mlir] Silence -Wdangling-assignment-gsl in OperationSupport.h"

This warning is causing lots of build spam when I use a recent Clang as
my host compiler. It's a potential false positive, so silence it until
llvm#126600 is resolved.

Reland of llvm#126140 with fix for
non-Clang compilers (the preprocessor doesn't short-circuit conditionals
the way I thought it did).

mlir/include/mlir/IR/OperationSupport.h

@@ -997,13 +997,25 @@ struct OperationState {
     if (!properties) {
       T *p = new T{};
       properties = p;
+#if defined(__clang__)
+#if __has_warning("-Wdangling-assignment-gsl")
+#pragma clang diagnostic push
+// https://github.com/llvm/llvm-project/issues/126600
+#pragma clang diagnostic ignored "-Wdangling-assignment-gsl"
+#endif
+#endif
       propertiesDeleter = [](OpaqueProperties prop) {
         delete prop.as<const T *>();
       };
-      propertiesSetter = [](OpaqueProperties new_prop,
+      propertiesSetter = [](OpaqueProperties newProp,
                             const OpaqueProperties prop) {
-        *new_prop.as<T *>() = *prop.as<const T *>();
+        *newProp.as<T *>() = *prop.as<const T *>();
       };
+#if defined(__clang__)
+#if __has_warning("-Wdangling-assignment-gsl")
+#pragma clang diagnostic pop
+#endif
+#endif
       propertiesId = TypeID::get<T>();
     }
     assert(propertiesId == TypeID::get<T>() && "Inconsistent properties");