Fix ELFObjectWriter::isLocal for signature symbols.

And with that simplify the logic for inserting them in ExternalSymbolData or
LocalSymbolData.

No functionality change overall since the old code avoided the isLocal bug.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238555 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed