Revert r254592 (virtual dtor in SCEVPredicate).

Clang has better diagnostics in this case.  It is not necessary therefore
to change the destructor to avoid what is effectively an invalid warning
in gcc.  Instead, better handle the warning flags given to the compiler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255905 91177308-0d34-0410-b5e6-96231b3b80d8