CFE support for nonfunction type aliases

[johnniwinther]

No description provided.

[eernstg]

@johnniwinther, I think #36870 should be removed from this epic, because it is concerned with an issue that arises without any use of non-function type aliases (so it shouldn't be blocked on experiment nonfunction-type-aliases).

I removed it from this epic. It is still a member of #41991 CFE dysfunctionalities. Please take a look if it needs to be added to some other epic.

[eernstg]

@johnniwinther, I think the non-function type aliases in the CFE are working, so I'll close this issue. There may still be bugs, of course, but the implementation passes all tests and has been stable for a while.

(Exception: language/nonfunction_type_aliases/mixed/aliased_cyclic_superclass_error_test fails, but that's because of a test runner issue, #44990.)