[PECOFF] Better handling of /OPT option.

Previously LLD would fail if /OPT:icf, /OPT:lbr or such are specified,
because these command line flags would be handled as unknown ones. We
rather want LLD to ignore these known but yet-to-be-implemented options
for now.

Added tests for the driver as well.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206863 91177308-0d34-0410-b5e6-96231b3b80d8