ELF: Undefine all symbols, not just those that we expect to be defined.

This allows the combined LTO object to provide a definition with the same
name as a symbol that was internalized without causing a duplicate symbol
error. This normally happens during parallel codegen which externalizes
originally-internal symbols, for example.

In order to make this work, I needed to relax the undefined symbol error to
only report an error for symbols that are used in regular objects.

Differential Revision: http://reviews.llvm.org/D19954

llvm-svn: 268649