What would a Christian who believes that there is sufficient salvific truth in all denominations and that the denomination-specific doctrines are irrelevant be called? Nondenominational Christian, lukewarm or something else?
Least-common-denominator Christian... or, depending upon what you count as a legitimate denomination, too-watered-down-to-be-an-actual-Christian. It's good to be gracious and charitable with the various traditions out there, but if you don't have any theological legs left to stand on then how can you stay safely on the foundation of Jesus Christ?
You could call them non-denominational. Another could be pain-in-the-bum. That is pain in their bum caused by the splinters from the wooden fence they insist on sitting on.
The risk of this position was highlighted by Bishop William Broughton who said that when you say the denomination does not matter then you are saying that the religion does not matter. This is quite different from believing that most/all denominations have some grasp of the truth, and many/most of them make a point of some value that has meaning for those who walk that way. As one who believes that we are not saved by the purity of our doctrine (and that is not saying that doctrine is not important) but by Christ Jesus and his atoning death and resurrection, I still recognise that I need to belong, and I need to have some views on some of these important issues. I would call a Christian who believes that there is sufficient salvific truth in many/most denominations Christian, and that many denomination-specific doctrines are irrelevant to the ultimate outcome, they are still important in our walk with Christ. The real grace we need to aspire to, is being confident in where we stand to be ready to share that, and the learn from other Christians without eroding that faith delivered to us.
I like Botolph's answer quite well. Let me add that some denominations really can't be considered truly "Christian" despite their names, because their base doctrines are simply heretical. For example: Jehovah's Witnesses, Latter Day Saints, Christian Scientists. Any church that does not teach its members the core truth of salvation solely by God's grace through faith in Jesus Christ is not a Christian group no matter what they call themselves.