Fix PR26085 by suppressing diags from CheckSingleAssignmentConstraints during overload resolution.
It seems that passing Diagnose/Complain flags around to Sema methods is a mildly common thing, so that’s the approach I took when making this. I’m certainly open to suggestions if there’s a better way to go about this, because "check a flag every time you call Diag" seems a bit fragile/error-prone.
N.B. The attached test case is the most ObjC I’ve ever written. If the added test cases seem unintuitive, please let me know. :)