[clang-tools-extra] [clang-tidy] Fix `DefaultHungarianPrefix` for incomplete classes in `readability-identifier-naming` (PR #215734)

[Zeyi Xu via cfe-commits]

================
@@ -416,6 +416,9 @@ IdentifierNamingCheck::~IdentifierNamingCheck() = default;
 
 bool IdentifierNamingCheck::HungarianNotation::checkOptionValid(
     int StyleKindIndex) const {
+  if (StyleKindIndex == SK_Default)
----------------
zeyi2 wrote:

Without this, when set `DefaultHungarianPrefix` to `On`, clang-tidy would give this warning:

```
warning: invalid identifier naming option 'DefaultHungarianPrefix' [clang-tidy-config]
```

Reproducer: https://clang-tidy.godbolt.org/z/h8Erej85q

https://github.com/llvm/llvm-project/pull/215734