Removing static initializer from Debug.cpp by converting to a ManagedStatic.

This is part of our larger effort to remove static initializers from LLVM libraries.

Reviewed by: chandlerc

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217053 91177308-0d34-0410-b5e6-96231b3b80d8