Prevent use of the implicit copy constructor on SmallPtrSetImpl. An accidental copy caused my SmallPtrSet->SmallPtrSetImpl conversion commit to fail the other day.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215971 91177308-0d34-0410-b5e6-96231b3b80d8
Const-correct and prevent a copy of a SmallPtrSet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215973 91177308-0d34-0410-b5e6-96231b3b80d8