[CSSolver] DynamicMemberLookup: Ignore disabled choices while filtering disjunctions

`filterDisjunction` should ignore the choices that are already
disabled while attempting to optimize disjunctions related to
dynamic member lookup.

Resolves: rdar://139314763

Author: xedin

lib/Sema/CSSolver.cpp

@@ -1781,6 +1781,9 @@ ConstraintSystem::filterDisjunction(
         return SolutionKind::Unsolved;
 
       for (auto *currentChoice : disjunction->getNestedConstraints()) {
+        if (currentChoice->isDisabled())
+          continue;
+
         if (currentChoice != choice)
           solverState->disableConstraint(currentChoice);
       }

validation-test/Sema/type_checker_crashers_fixed/rdar139314763.swift

@@ -0,0 +1,33 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+class A {}
+
+struct B {}
+
+@dynamicMemberLookup
+enum DynMember {
+  subscript<T>(_: T.Type) -> T {
+    get { fatalError() }
+  }
+
+  subscript<T>(dynamicMember keyPath: KeyPath<B, T>) -> T {
+    fatalError()
+  }
+}
+
+@dynamicMemberLookup
+class Test {
+  subscript<T>(_: KeyPath<A, T>) -> T {
+    self[setting: T.self]
+  }
+
+  subscript<T>(setting: T.Type) -> T {
+    get {
+      fatalError()
+    }
+  }
+
+  subscript<T>(dynamicMember keyPath: KeyPath<DynMember, T>) -> T {
+    fatalError()
+  }
+}