Fix the tests for forward declared union pointer as return type by making the return type pointer _Nonnull (#5878)

Followup of #5773.
Part of #5772.

Author: bricknerb

toolchain/check/testdata/interop/cpp/function/union.carbon

@@ -394,23 +394,16 @@ fn F() {
 
 union U;
 
-auto foo() -> U*;
+auto foo() -> U* _Nonnull;
 
-// --- fail_todo_import_decl_pointer_return_type.carbon
+// --- import_decl_pointer_return_type.carbon
 
 library "[[@TEST_NAME]]";
 
 import Cpp library "decl_pointer_return_type.h";
 
 fn F() {
   //@dump-sem-ir-begin
-  // CHECK:STDERR: fail_todo_import_decl_pointer_return_type.carbon:[[@LINE+7]]:3: error: semantics TODO: `Unsupported: nullable pointer: U *` [SemanticsTodo]
-  // CHECK:STDERR:   Cpp.foo();
-  // CHECK:STDERR:   ^~~~~~~
-  // CHECK:STDERR: fail_todo_import_decl_pointer_return_type.carbon:[[@LINE+4]]:3: note: in `Cpp` name lookup for `foo` [InCppNameLookup]
-  // CHECK:STDERR:   Cpp.foo();
-  // CHECK:STDERR:   ^~~~~~~
-  // CHECK:STDERR:
   Cpp.foo();
   //@dump-sem-ir-end
 }
@@ -974,9 +967,11 @@ fn F() {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_todo_import_decl_pointer_return_type.carbon
+// CHECK:STDOUT: --- import_decl_pointer_return_type.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %U: type = class_type @U [concrete]
+// CHECK:STDOUT:   %ptr: type = ptr_type %U [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
 // CHECK:STDOUT: }
@@ -997,7 +992,7 @@ fn F() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   %foo.ref: %foo.type = name_ref foo, imports.%foo.decl [concrete = constants.%foo]
-// CHECK:STDOUT:   %foo.call: init <error> = call %foo.ref()
+// CHECK:STDOUT:   %foo.call: init %ptr = call %foo.ref()
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT: