COFF: Support /force flag.

This option is to ignore remaining undefined symbols and force
the linker to create an output file anyways.

The existing code assumes that there's no undefined symbol after
reportRemainingUndefines(). That assumption is legitimate.
I also don't want to mess up the existing code for this minor feature.
In order to keep it as is, remaining undefined symbols are replaced
with dummy defined symbols.

llvm-svn: 240913