Fix comments of createReplaceableForwardDecl() and createForwardDecl().

Noticed while trying to understand how the merge of forward decalred types
and defintions work.

Reviewers: echristo, dblaikie, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5291

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217514 91177308-0d34-0410-b5e6-96231b3b80d8