[ADT] Move IsSizeLessThanThresholdT into AdjustedParamTBase (NFC) (#159900)

This patch moves IsSizeLessThanThresholdT into AdjustedParamTBase, the
sole user of the helper, while switching to a type alias.

Aside from moving the helper closer to where it's used, another
benefit is that we can assume that T is a complete type inside
AdjustedParamTBase.  Note that sizeof(T) serves as a check for a
complete type.  Inside AdjustedParamTBase, we only pass complete
non-void types to:

  std::is_trivially_copy_constructible<T>
  std::is_trivially_move_constructible<T>

so we can safely drop the fallback case implemented with
std::false_type.

Author: kazutakahirata

llvm/include/llvm/ADT/FunctionExtras.h

@@ -82,13 +82,6 @@ template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
   static constexpr size_t InlineStorageSize = sizeof(void *) * 3;
   static constexpr size_t InlineStorageAlign = alignof(void *);
 
-  template <typename T, class = void>
-  struct IsSizeLessThanThresholdT : std::false_type {};
-
-  template <typename T>
-  struct IsSizeLessThanThresholdT<
-      T, std::enable_if_t<sizeof(T) <= 2 * sizeof(void *)>> : std::true_type {};
-
   // Provide a type function to map parameters that won't observe extra copies
   // or moves and which are small enough to likely pass in register to values
   // and all other types to l-value reference types. We use this to compute the
@@ -101,6 +94,9 @@ template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
   template <typename T> struct AdjustedParamTBase {
     static_assert(!std::is_reference<T>::value,
                   "references should be handled by template specialization");
+    template <typename U>
+    using IsSizeLessThanThresholdT =
+        std::bool_constant<sizeof(U) <= 2 * sizeof(void *)>;
     using type =
         std::conditional_t<std::is_trivially_copy_constructible<T>::value &&
                                std::is_trivially_move_constructible<T>::value &&