Revert r301897, "ELF: Set symbol binding to STB_GLOBAL when undefining symbols during LTO."

It doesn't matter what binding we store in a non-UsedInRegularObj undefined
symbol because we should reset it when we see a real undefined symbol in
a combined LTO object. The fact that we weren't doing so before is a bug
(PR32899) which is now fixed.

llvm-svn: 302067