[PECOFF] Ignore /incremental option.

/incremental is an option to enable incremental linking. We will eventually
want to implement the feature for better performance, but in the meantime,
we want to just ignore the option so that the linker does not output unknown
option error when it sees /incremental option.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191063 91177308-0d34-0410-b5e6-96231b3b80d8