Sema: improve behavior of comptime_int backed enums

test/cases/compile_errors/enum_backed_by_comptime_int_must_be_comptime.zig

@@ -0,0 +1,11 @@
+pub export fn entry() void {
+    const E = enum(comptime_int) { a, b, c, _ };
+    var e: E = .a;
+    _ = e;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:12: error: variable of type 'tmp.entry.E' must be const or comptime