Sema: check for astgen failures in semaStructFields

The struct might be a top level struct in which case it might not have Zir.

Closes #12548

test/compile_errors.zig

@@ -184,7 +184,7 @@ pub fn addCases(ctx: *TestContext) !void {
     }
 
     {
-        const case = ctx.obj("argument causes error ", .{});
+        const case = ctx.obj("argument causes error", .{});
         case.backend = .stage2;
 
         case.addSourceFile("b.zig",
@@ -208,6 +208,23 @@ pub fn addCases(ctx: *TestContext) !void {
         });
     }
 
+    {
+        const case = ctx.obj("astgen failure in file struct", .{});
+        case.backend = .stage2;
+
+        case.addSourceFile("b.zig",
+            \\bad
+        );
+
+        case.addError(
+            \\pub export fn entry() void {
+            \\    _ = (@sizeOf(@import("b.zig")));
+            \\}
+        , &[_][]const u8{
+            ":1:1: error: struct field missing type",
+        });
+    }
+
     // TODO test this in stage2, but we won't even try in stage1
     //ctx.objErrStage1("inline fn calls itself indirectly",
     //    \\export fn foo() void {