[Test] Reproducer for witness table deserialization failure with a enum_flag

Author: xymus

test/Serialization/Recovery/Inputs/custom-modules/Conformance.h

@@ -0,0 +1,4 @@
+enum __attribute__((flag_enum,enum_extensibility(open))) CEnum : int {
+  A = 1 << 0,
+  B = 1 << 1,
+};

test/Serialization/Recovery/Inputs/custom-modules/module.modulemap

@@ -13,3 +13,4 @@ module SuperclassObjC { header "SuperclassObjC.h" }
 module Typedefs { header "Typedefs.h" }
 module TypeRemovalObjC { header "TypeRemovalObjC.h" }
 module Types { header "Types.h" }
+module Conformance { header "Conformance.h" }

test/Serialization/Recovery/missing-clang-module-conformance.swift

@@ -0,0 +1,18 @@
+/// Recover from reading witness table involving a synthesized conformance
+/// rdar://problem/58924131
+
+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/custom-modules %t/
+// RUN: %target-swift-frontend -emit-module %s -module-name MyModule -emit-module-path %t/MyModule.swiftmodule -I %t/custom-modules -swift-version 5
+
+/// Delete the clang module
+// RUN: rm -r %t/custom-modules/
+
+// RUN: not %target-sil-opt %t/MyModule.swiftmodule 2>&1 | %FileCheck %s
+
+@_implementationOnly import Conformance
+// CHECK: missing required module 'Conformance'
+
+public func foo<T: OptionSet>(_ t: T) {}
+
+foo(CEnum.A)