[Modules] Fix potential ODR violations by sinking the DEBUG_TYPE
definition below all of the header #include lines, Polly edition.

If you want to know more details about this, you can see the recent
commits to Debug.h in LLVM. This is just the Polly segment of a cleanup
I'm doing globally for this macro.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@206852 91177308-0d34-0410-b5e6-96231b3b80d8