sema: add compile error for duplicate struct field

test/compile_errors/stage2/struct_duplicate_field_name.zig

@@ -0,0 +1,15 @@
+const S = struct {
+    foo: u32,
+    foo: u32,
+};
+
+export fn entry() void {
+    const s: S = .{ .foo = 100 };
+    _ = s;
+}
+
+// duplicate struct field name
+//
+// :3:5: error: duplicate struct field: 'foo'
+// :2:5: note: other field here
+// :1:11: note: struct declared here