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

If you want more details about this, you can see some of the commits to
Debug.h in LLVM recently. This is just the clang section of a cleanup
I've done for all uses of DEBUG_TYPE in LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206849 91177308-0d34-0410-b5e6-96231b3b80d8