[clang] [ASTMatchers] Simplify isDefaultedHelper (NFC) (PR #137571)

[Jakub Kuderski via cfe-commits]

================
@@ -871,14 +871,11 @@ IteratorT matchesFirstInPointerRange(const MatcherT &Matcher, IteratorT Start,
   return End;
 }
 
-template <typename T, std::enable_if_t<!std::is_base_of<FunctionDecl, T>::value>
-                          * = nullptr>
-inline bool isDefaultedHelper(const T *) {
+template <typename T> inline bool isDefaultedHelper(const T *FD) {
+  if constexpr (std::is_base_of<FunctionDecl, T>::value)
----------------
kuhar wrote:

```suggestion
  if constexpr (std::is_base_of_v<FunctionDecl, T>)
```

https://github.com/llvm/llvm-project/pull/137571