'FoldingSetNodeImpl' (previously 'FoldingSetNodeID' was a typedef of
'FoldingSetNodeImpl::NodeID').
Why? Clients can now easily forward declare 'FoldingSetNodeID' without having
to include FoldingSet.h.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46187 91177308-0d34-0410-b5e6-96231b3b80d8