[Clang] Fix compat diagnostic to detect a nontype template parameter has a...

[Clang] Fix compat diagnostic to detect a nontype template parameter has a placeholder type using getContainedAutoType()

Based on the changes introduced by 15361a21 it
looks like C++17 compatibility diagnostic should have been checking
getContainedAutoType().

This fixes: https://github.com/llvm/llvm-project/issues/57369
  https://github.com/llvm/llvm-project/issues/57643
  https://github.com/llvm/llvm-project/issues/57793

Differential Revision: https://reviews.llvm.org/D132990