Fix 'gcc -flto' builds for unittest binaries (undefined reference to
`typeinfo for llvm::cl::GenericOptionValue').

Remove an "anchor" method for an abstract class. (This does not
increase the number of vtables.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183830 91177308-0d34-0410-b5e6-96231b3b80d8