Provide a version of getSymbolOffset that returns false on error.

This simplifies ELFObjectWriter::SymbolValue a bit more. This new version
will also be used in the COFF writer to fix pr19147.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207711 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed