GenericCloner: Generate a valid debug location if no decl is available for a SILArgument

Without this we used to crash in a test case in IRGen trying to generate code
for an invalid debug location.

rdar://87565600

lib/SILOptimizer/Utils/GenericCloner.cpp

@@ -72,7 +72,9 @@ void GenericCloner::populateCloned() {
   SmallVector<SILValue, 4> entryArgs;
   entryArgs.reserve(OrigEntryBB->getArguments().size());
   for (auto &OrigArg : OrigEntryBB->getArguments()) {
-    RegularLocation Loc((Decl *)OrigArg->getDecl());
+    RegularLocation Loc = OrigArg->getDecl() ?
+      RegularLocation((Decl *)OrigArg->getDecl()) :
+      RegularLocation::getAutoGeneratedLocation();
     AllocStackInst *ASI = nullptr;
     SILType mappedType = remapType(OrigArg->getType());
 

test/DebugInfo/generic-specializer.swift

@@ -0,0 +1,32 @@
+// RUN: %target-swift-frontend %s -O -enable-library-evolution -emit-ir -g
+
+// REQUIRES: objc_interop
+import Foundation
+
+public struct MyThing {}
+
+public struct ThingSequence: Sequence, IteratorProtocol {
+  private var enumerator: NSEnumerator?
+
+  public init() { }
+
+  public mutating func next() -> MyThing? {
+    guard let enumerator = enumerator else { return nil }
+    guard let nextObject = enumerator.nextObject(),
+          let nextThing = nextObject as? MyThing else {
+      self.enumerator = nil
+      return nil
+    }
+    return nextThing
+  }
+}
+
+public struct Manager {
+  public func sequence() -> ThingSequence {
+    ThingSequence()
+  }
+}
+
+public func test(m: Manager) {
+  for _ in m.sequence() { }
+}