[dsymutil] Discard useless location attributes.

When cloning the debug info for a function that hasn't been linked,
strip the DIEs from all location attributes that wouldn't contain any
meaningful information anyway.

This kind of situation can happen when a function got discarded by the
linker, but its debug information is still wanted in the final link
because it was marked as required as some other DIE dependency. The easiest
way to get into that situation is to have using directives. They get
linked unconditionally, but their targets might not always be present.

llvm-svn: 247386