Sema: validate @intToEnum int operand type

test/cases/compile_errors/duplicate_enum_field.zig

@@ -0,0 +1,16 @@
+const Foo = enum {
+    Bar,
+    Bar,
+};
+
+export fn entry() void {
+    const a: Foo = undefined;
+    _ = a;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:5: error: duplicate enum field 'Bar'
+// :2:5: note: other field here