Fix GH-18108 gen_stub: Using $this when not in object context

[kocsismate]

Fix for #18108

Reproducer in ./ext/zend_test/test.stub.php:

 /** @var string */
const TEMP_TEST = __FILE__;

The new error message will be the following:

In ./ext/zend_test/test.stub.php:
Expression at line 12 must be a global, non-magic constant

[TimWolla]

@kocsismate Prefer Squash and Merge for single-commit PRs and when the commit history of the PR is not important. Squash and Merge will apply GitHub's PGP signature on the squashed commit, whereas Rebase and Merge will not, resulting in an unsigned commit.

[kocsismate]

@TimWolla Ah sorry, I made the same mistake a few weeks ago... I will try hard not to do it again :)

BTW I used "rebase and merge" because it was the default for me, and I thought there would be no difference between the two options in case of this single commit PR...