Revert "Apply clang-tidy fixes for readability-redundant-declaration in Debug.cpp (NFC)"

This reverts commit 0fe01a9346658c0955b68b123f2b470b018114b1.

The commit caused build failures like:

  llvm/lib/Support/Debug.cpp:65:3: error: ‘setCurrentDebugTypes’ was
  not declared in this scope; did you mean ‘setCurrentDebugType’?