[IRGen] Pass component generic sig when emitting key path component f… (#64871)

* [IRGen] Pass component generic sig when emitting key path component for external property

rdar://101179225

When no generic environment was present, we passed nullptr, which is not correct when the property uses an external associated type, causing crashes in IRGen. In those cases, we have to pass the component generic sig instead.

* Fix test

Author: drexin

lib/IRGen/GenKeyPath.cpp

@@ -904,7 +904,8 @@ emitKeyPathComponent(IRGenModule &IGM,
                     reqt.getProtocol());
                 externalSubArgs.push_back(IGM.emitWitnessTableRefString(
                     substType, conformance,
-                    genericEnv ? genericEnv->getGenericSignature() : nullptr,
+                    genericEnv ? genericEnv->getGenericSignature() :
+                                 componentCanSig,
                     /*shouldSetLowBit*/ true));
               }
             });

validation-test/IRGen/rdar101179225.swift

@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -disable-availability-checking %s -emit-ir > /dev/null
+
+struct Section: Identifiable {
+    let id: Int
+    let items: [any Item]
+}
+
+protocol Item: AnyObject, Identifiable {
+}
+
+protocol P {
+}
+
+struct Iter<D, I, C>: P where D : RandomAccessCollection, I : Hashable {
+    init(_ xs: D, id: KeyPath<D.Element, I>, f: (D.Element) -> C) {}
+}
+
+struct V {
+  var s: Section
+  var p: some P {
+    Iter(s.items, id: \.id) { x in
+      ""
+    }
+  }
+}