Auto merge of #51196 - Havvy:remove-keywords, r=petrochenkov

Implement RFC 2421, 'Keyword unreservations (pure, sizeof, alignof, offsetof)

On my local machine I was getting failures in `test/ui-fulldeps`, but I was also getting them again after reverting the change, so I'm hoping that this works.

I removed the test because that's what the other PR that unreserved a keyword did and I feel that it doesn't make sense/keep value to keep a test for something removed years ago.