[Types] Fix missing inverse check for described field in isValidSupertype (#8378)

## Summary

- Add the missing `else` branch in `isValidSupertype` for the
`described` field check, making it symmetric with the existing
`descriptor` field check.
- Without this fix, a type without a `describes` clause can illegally
subtype a type that has a `describes` clause (i.e., a descriptor type),
as long as all features are enabled.
- The `descriptor` check already handles both directions (if sub has no
descriptor, super must also have no descriptor), but the `described`
check was missing the inverse direction.

## Details

In `isValidSupertype` (src/wasm/wasm-type.cpp), the validation for
`descriptor` is fully symmetric:

```cpp
if (sub.descriptor) {
    if (super.descriptor && sub.descriptor->supertype != super.descriptor) {
        return false;
    }
} else {
    if (super.descriptor) { return false; }
}
```

But the `described` check was missing the else branch:

```cpp
if (sub.described) {
    if (!super.described || sub.described->supertype != super.described) {
        return false;
    }
}
// Missing: else { if (super.described) { return false; } }
```

The spec test at `test/spec/descriptors.wast` (lines 152-161) already
covers this case with an `assert_invalid` for "supertype of
non-descriptor type cannot be a descriptor". However, the test currently
passes for the wrong reason: `wasm-shell` re-parses `assert_invalid`
modules as quoted text modules without features enabled, so
`TypeBuilder::build()` rejects the module with
`RequiresCustomDescriptors` before ever reaching `isValidSupertype`.
When the module is loaded through the normal inline parse path (with all
features enabled), the invalid subtyping relationship is incorrectly
accepted.

## Test plan

- Verified that `wasm-shell test/spec/descriptors.wast` passes all 14
checks (including the one at line 152)
- Verified that all 309 GTest unit tests pass

Author: sumleo

src/tools/wasm-shell.cpp

@@ -102,6 +102,7 @@ struct Shell {
     std::shared_ptr<Module> wasm;
     if (auto* quoted = std::get_if<QuotedModule>(&mod.module)) {
       wasm = std::make_shared<Module>();
+      wasm->features = FeatureSet::All;
       switch (quoted->type) {
         case QuotedModuleType::Text: {
           CHECK_ERR(parseModule(*wasm, quoted->module));

src/wasm/wasm-type.cpp

@@ -2418,6 +2418,12 @@ bool isValidSupertype(const HeapTypeInfo& sub, const HeapTypeInfo& super) {
     if (!super.described || sub.described->supertype != super.described) {
       return false;
     }
+  } else {
+    // A supertype of a type without a describes clause must also not have a
+    // describes clause.
+    if (super.described) {
+      return false;
+    }
   }
   SubTyper typer;
   switch (sub.kind) {