[ADT] Allow IsSizeLessThanThresholdT for incomplete types. NFC

If the type T is incomplete then sizeof(T) results in C++ compilation error at line:
  static constexpr bool value = sizeof(T) <= (2 * sizeof(void *));

This patch allows incomplete types in parameters of function. Example:
  using SomeFunc = void(SomeIncompleteType &);
  llvm::unique_function<SomeFuncType> SomeFunc;

Reviewers: DaniilSuchkov, vvereschaka

Differential Revision: https://reviews.llvm.org/D81554