[PECOFF] Ignore /ignore option.

/ignore:<number> is a linker option to disable warning specified by
the number. We ignore the option because it does not make sense for
LLD.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206636 91177308-0d34-0410-b5e6-96231b3b80d8