[AST] Handle getSuperclassForDecl() returning an ErrorType in more places

[nathawes]

We already were at some call sites, but weren't in ModuleDecl::lookupConformance() or TypeBase::getContextSubstitutions(). Code completion was hitting both of these cases.

Slava fixed the root cause of this in #26776. This is now just adding a regression test.

Resolves rdar://problem/54145042

[nathawes]

@swift-ci please test

[slavapestov]

Instead of fixing every call site, I think its better to address the root cause, which is that getSuperclass() returns an empty type if the substitution fails. It should use SubstFlags::UseErrorType instead. Try this change: 70b5342

[nathawes]

Thanks @slavapestov! Adding the flag worked. I had to change the helper a little from what was in your patch to maintain the existing behavior of getSuperclass(). Does this look ok now? Should I try removing the other checks for ErrorType?

[nathawes]

@swift-ci please test

[swift-ci]

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[swift-ci]

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[slavapestov]

@nathawes I went ahead and changed Type::subst() so that it always returns a non-null Type, eliminating SubstFlags::UseErrorType in the process: #26776

Your new test case should pass with no further changes once my PR lands.

[nathawes]

@swift-ci please test

[swift-ci]

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[swift-ci]

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[nathawes]

@swift-ci please test

[nathawes]

@swift-ci please smoke test

[swift-ci]

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[swift-ci]

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