Add regression test for https://bugs.swift.org/browse/SR-15917

Author: slavapestov

test/SILGen/type_lowering_subst_function_type_caseiterable.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-emit-silgen %s -requirement-machine-abstract-signatures=on | %FileCheck %s
+
+// This is a GenericSignatureBuilder bug fixed with the Requirement Machine, from
+// https://bugs.swift.org/browse/SR-15917.
+
+public struct Foo<Unused: CaseIterable> {
+  public struct Nested {}
+
+  public let closure: () -> Nested
+}
+
+// CHECK-LABEL: sil [transparent] [serialized] [ossa] @$s029type_lowering_subst_function_A13_caseiterable3FooV7closureAC6NestedVyx_Gycvg : $@convention(method) <Unused where Unused : CaseIterable> (@guaranteed Foo<Unused>) -> @owned @callee_guaranteed @substituted <τ_0_0, τ_0_1 where τ_0_0 : CaseIterable, τ_0_0 == τ_0_1> () -> Foo<τ_0_0>.Nested for <Unused, Unused>