[Type checker] Don't substitute into unresolved types.

[DougGregor]

Unresolved types are formed in a few specific places within the type
checker's recovery path; don't let them bleed into the substitution
logic. Fixes rdar://problem/42448618.

[DougGregor]

@swift-ci please smoke test and merge

[slavapestov]

This looks like a workaround. What's the actual problem?

[slavapestov]

I see, we're building a new generic signature because getTypeOfMember() calls subst() on a GenericFunctionType. Should it really be doing that?

[DougGregor]

subst() on a GenericFunctionType is defined, but probably generally a bad idea. UnresolvedType is the real issue here; anywhere it leaks into the substitution machinery we risk hitting this problem.