Comment 43 for bug 1332900

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/commit/sound/pci/hda?id=73bdd597823e2231dc882577dbbaf8df92fe1775

the other dell notebook with one combo jack support headset, headphone or mic

if your alienware 18 support headset , node 0x1b is most likely the headset mic pin but you don't need headphone mic pin since your alienware 18 have mic jack

  },
+ [ALC668_FIXUP_DELL_MIC_NO_PRESENCE] = {
+ .type = HDA_FIXUP_PINS,
+ .v.pins = (const struct hda_pintbl[]) {
+ { 0x19, 0x03a1913d }, /* use as headphone mic, without its own jack detect */
+ { 0x1b, 0x03a1113c }, /* use as headset mic, without its own jack detect */
+ { }
+ },
+ .chained = true,
+ .chain_id = ALC668_FIXUP_HEADSET_MODE
+ },