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’?

llvm/lib/Support/Debug.cpp

@@ -61,6 +61,12 @@ bool isCurrentDebugType(const char *DebugType) {
   return false;
 }
 
+/// Set the current debug type, as if the -debug-only=X
+/// option were specified.  Note that DebugFlag also needs to be set to true for
+/// debug output to be produced.
+///
+void setCurrentDebugTypes(const char **Types, unsigned Count);
+
 void setCurrentDebugType(const char *Type) {
   setCurrentDebugTypes(&Type, 1);
 }