Don't emit an internal destructor that is identical to an external one.

It is not safe to emit alias to undefined (not supported by ELF or COFF), but
it is safe to rauw when the alias would have been internal or linkonce_odr.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@194307 91177308-0d34-0410-b5e6-96231b3b80d8