Revert "[Sema] ban multi-arguments to tuple coercion"

[jrose-apple]

Reverts #3895. It breaks cases where there really is a single unlabeled argument of tuple type, like this:

let pairs = [(1, "A"), (2, "B")]
print(pairs.map { $0.0 })

[jrose-apple]

@swift-ci Please smoke test

[jrose-apple]

This should be followed by reverts of the downstream patches swiftlang/swift-corelibs-foundation#497, swiftlang/swift-corelibs-xctest#149, and swiftlang/swift-package-manager#581; the changes there are incorrect for higher-order functions.

[jrose-apple]

cc @dduan and @DougGregor (to keep everyone in the loop)

[dduan]

👍