Really ignore -Wgnu-variable-sized-type-not-at-end (#3627)

(cherry picked from commit 56863e8f0b)

src/backend/distributed/planner/shard_pruning.c

@@ -151,6 +151,23 @@ typedef union \
 typedef FunctionCallInfoData FunctionCall2InfoData;
 #endif
 
+/*
+ * We also ignore this warning in ./configure, but that's not always enough.
+ * The flags that are used during compilation by ./configure are determined by
+ * the compiler support it detects. This is usually GCC. This warning is only
+ * present in clang. So it would normally be fine to not use it with GCC. The
+ * problem is that clang is used to compile the JIT bitcode when postgres is
+ * compiled with -with-llvm. So in the end both clang and GCC are used to
+ * compile the project.
+ *
+ * So the flag is not provided on the command line, because ./configure notices
+ * that GCC doesn't support it. But this warning persists when compiling the
+ * bitcode. So that's why we ignore it here explicitly.
+ */
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wgnu-variable-sized-type-not-at-end"
+#endif /* __clang__ */
+
 /*
  * Data necessary to perform a single PruneShards().
  */