[Serialization] Do not assert when inherited type is null

[bnbarham]

When compiling with allow errors, it's possible to have invalid
inherited types - both null and ErrorType.

Cleaned up the tests a little - moved the majority of
Frontend/allow-errors.swift into separate files in
Serialization/AllowErrors and use split_file.py instead of #defines.

Resolves rdar://78048470

[bnbarham]

@xymus what are your thoughts on the asserts? Should I add a check for whether compiling with errors or leave as is?

[bnbarham]

@swift-ci please test

[xymus]

Yeah, I think it would be worth taking into account AllowModuleWithCompilerErrors in the asserts. It can prevent us from introducing future unrelated regressions.

[bnbarham]

Yeah, I think it would be worth taking into account AllowModuleWithCompilerErrors in the asserts. It can prevent us from introducing future unrelated regressions.

While updating this I realised that addTypeRef already has a null type + error check, so adding another would just be duplicating it.