Fix bugs in st_entry, and in tests

Author: elsteveogrande

libcxx/include/__stacktrace/entry.h

@@ -62,7 +62,7 @@ class _LIBCPP_EXPORTED_FROM_ABI stacktrace_entry : private __stacktrace::entry_b
     return {};
   }
   [[nodiscard]] _LIBCPP_EXPORTED_FROM_ABI string source_file() const {
-    if (__desc_.has_value()) {
+    if (__file_.has_value()) {
       return {__file_->data(), __file_->size()};
     }
     return {};

libcxx/test/std/diagnostics/stacktrace/entry.cons/copy_assign.pass.cpp

@@ -27,7 +27,7 @@ _LIBCPP_NO_TAIL_CALLS
 int main(int, char**) {
   static_assert(std::is_nothrow_copy_assignable_v<std::stacktrace_entry>);
 
-  std::stacktrace_entry e1 = std::stacktrace::current()[0];
+  auto& e1 = std::stacktrace::current()[0];
   std::stacktrace_entry e2;
   static_assert(noexcept(e2 = e1));
   e2 = e1;

libcxx/test/std/diagnostics/stacktrace/entry.cons/copy_construct.pass.cpp

@@ -27,7 +27,7 @@ _LIBCPP_NO_TAIL_CALLS
 int main(int, char**) {
   static_assert(std::is_nothrow_copy_constructible_v<std::stacktrace_entry>);
 
-  std::stacktrace_entry e1 = std::stacktrace::current()[0];
+  auto& e1 = std::stacktrace::current()[0];
   static_assert(noexcept(std::stacktrace_entry(e1)));
   std::stacktrace_entry e2(e1);
   assert(e2 == e1);