[Clang] Fix lambda CheckForDefaultedFunction(...) so that it checks the...

[Clang] Fix lambda CheckForDefaultedFunction(...) so that it checks the CXXMethodDecl is a special member function before attempting to call DefineDefaultedFunction(...)

In Sema::CheckCompletedCXXClass(...) It used a lambda CheckForDefaultedFunction
the CXXMethodDecl passed to CheckForDefaultedFunction may not be a special
member function and so before attempting to apply functions that only apply to
special member functions it needs to check. It fails to do this before calling
DefineDefaultedFunction(...). This PR adds that check and test to verify we no
longer crash.

This fixes https://github.com/llvm/llvm-project/issues/57431

Differential Revision: https://reviews.llvm.org/D132906