Previously we had loosened the behavior so that function proc addresses are returned
for an extension function even if that extension is disabled, if the API was artificially
restricted during testing, to bypass CTS tests that appeared to assume certain
extension functions were available.
This loosening in turn caused the dEQP-VK.api.version_check.entry_points test to fail,
so the restriction is reapplied and only functions from enabled extensions are published.
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