Handle non-deserializable SIL functions referenced in witness tables

This is enough to let the test case in rdar://problem/40899824 pass,
and any callers of this function already need to be able to handle a
nullptr result. There's a lot more work to do in this area, but it's
nice to get the simple things working again.

Author: jrose-apple

lib/Serialization/DeserializeSIL.cpp

@@ -375,7 +375,15 @@ SILFunction *SILDeserializer::getFuncForReference(StringRef name) {
   if (iter == FuncTable->end())
     return nullptr;
 
-  return readSILFunction(*iter, nullptr, name, /*declarationOnly*/ true);
+  auto maybeFn = readSILFunctionChecked(*iter, nullptr, name,
+                                        /*declarationOnly*/ true);
+  if (!maybeFn) {
+    // Ignore the failure and just pretend the function doesn't exist
+    llvm::consumeError(maybeFn.takeError());
+    return nullptr;
+  }
+
+  return maybeFn.get();
 }
 
 /// Helper function to find a SILGlobalVariable given its name. It first checks

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

@@ -0,0 +1 @@
+module rdar40899824Helper { header "rdar40899824Helper.h" }

validation-test/Serialization/Inputs/custom-modules/rdar40899824Helper.h

@@ -0,0 +1,13 @@
+#ifndef BAD
+typedef struct {
+  int value;
+} SoonToBeMissing;
+#endif
+
+@interface Impl
+#ifndef BAD
+- (void)use:(SoonToBeMissing)value;
+#endif
+
+- (void)unrelated;
+@end

validation-test/Serialization/rdar40899824.swift

@@ -0,0 +1,38 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -emit-module -o %t/Library.swiftmodule -I %S/Inputs/custom-modules -DLIBRARY -Xfrontend -enable-objc-interop -Xfrontend -disable-objc-attr-requires-foundation-module
+// RUN: %target-swift-frontend %s -I %t -I %S/Inputs/custom-modules -enable-objc-interop -emit-ir > /dev/null
+
+// RUN: %target-swift-frontend %s -I %t -I %S/Inputs/custom-modules -enable-objc-interop -emit-ir -Xcc -DBAD > /dev/null
+// RUN: %target-swift-frontend %s -I %t -I %S/Inputs/custom-modules -enable-objc-interop -emit-ir -Xcc -DBAD -O > /dev/null
+
+#if LIBRARY
+
+import rdar40899824Helper
+
+public protocol Proto: class {
+  func use(_: SoonToBeMissing)
+  func unrelated()
+}
+
+extension Impl: Proto {}
+
+#else // LIBRARY
+
+import Library
+import rdar40899824Helper
+
+func testGeneric<T: Proto>(_ obj: T) {
+  obj.unrelated()
+}
+
+func testExistential(_ obj: Proto) {
+  obj.unrelated()
+}
+
+func test(_ proto: Proto, _ impl: Impl) {
+  impl.unrelated()
+  testGeneric(impl)
+  testExistential(impl)
+}
+
+#endif // LIBRARY