FoldingSet: Make FoldingSetImpl's dtor protected and non-virtual

It's not intended to be polymorphically deleted. Make FoldingSet
and ContextualFoldingSet final to avoid noise from -Wnon-virtual-dtor.

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232922 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/FoldingSet.cpp

@@ -223,6 +223,8 @@ static void **AllocateBuckets(unsigned NumBuckets) {
 //===----------------------------------------------------------------------===//
 // FoldingSetImpl Implementation
 
+void FoldingSetImpl::anchor() {}
+
 FoldingSetImpl::FoldingSetImpl(unsigned Log2InitSize) {
   assert(5 < Log2InitSize && Log2InitSize < 32 &&
          "Initial hash table size out of range");