Add documentation comment to ForwardTemplateReference.

This node doesn't directly correspond to a mangled name fragment, so
it's useful to explicitly describe when it's created and what it's for.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340664 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed