Clarify the code in checkDLLAttribute()

Update the comments to make it more clear what's going on, and address
Richard's comments from PR21718. This doesn't fix that bug, but hopefully
makes the code easier to understand.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224303 91177308-0d34-0410-b5e6-96231b3b80d8