Fix -Wdeprecated regarding ORC copying ValueMaterializers
As usual, this is a polymorphic hierarchy without polymorphic ownership,
so simply make the dtor protected non-virtual, protected default copy
ctor/assign, and make derived classes final. The derived classes will
pick up correct default public copy ops (and dtor) implicitly.
(wish I could add -Wdeprecated to the build, but last time I tried it
triggered on some system headers I still need to look into/figure out)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250747
91177308-0d34-0410-b5e6-
96231b3b80d8