Sema: disallow using stage1 fn ptrs in extern contexts

Closes ziglang#13022

compile_errors/old_fn_ptr_in_extern_context.zig

@@ -0,0 +1,20 @@
+const S = extern struct {
+    a: fn () callconv(.C) void,
+};
+comptime {
+    _ = @sizeOf(S) == 1;
+}
+comptime {
+    _ = [*c][4]fn() callconv(.C) void;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:5: error: extern structs cannot contain fields of type 'fn() callconv(.C) void'
+// :2:5: note: type has no guaranteed in-memory representation
+// :2:5: note: use '*const ' to make a function pointer type
+// :8:13: error: C pointers cannot point to non-C-ABI-compatible type '[4]fn() callconv(.C) void'
+// :8:13: note: type has no guaranteed in-memory representation
+// :8:13: note: use '*const ' to make a function pointer type