[modules] Stop excluding Support/Debug.h from the Support module. This header
has been modular since r206822, and excluding it was leading to workarounds
such as the one in r219592, which this change removes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219593 91177308-0d34-0410-b5e6-96231b3b80d8