Remove the unnecessary virtual dtor from the DIEUnit hierarchy (in favor of protected...

Remove the unnecessary virtual dtor from the DIEUnit hierarchy (in favor of protected dtor in the base, final derived classes with public non-virtual dtors)

These objects are never polymorphically owned/destroyed, so the virtual
dtor was unnecessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301068 91177308-0d34-0410-b5e6-96231b3b80d8