Add a reproducer for issue 78447

#78447

Author: hjyamauchi

test/Misc/Inputs/issue-78447-c.h

@@ -0,0 +1,3 @@
+typedef struct _xmlNs {
+    const char *prefix;
+} *xmlNsPtr;

test/Misc/Inputs/module.modulemap

@@ -0,0 +1,3 @@
+module Issue_78447_C {
+  header "issue-78447-c.h"
+}

test/Misc/issue-78447.swift

@@ -0,0 +1,20 @@
+// RUN: %target-build-swift %s -I %S/Inputs
+
+import Issue_78447_C
+
+func foo<T> (ptr: UnsafePointer<T>?) -> String? {
+  return nil
+}
+
+class A {
+  internal let ptr: xmlNsPtr
+  internal init(ptr: xmlNsPtr) {
+    self.ptr = ptr
+  }
+}
+
+class B : A {
+  func bar() -> String? {
+    return foo(ptr: ptr.pointee.prefix)
+  }
+}