[GSB] Don't try to infer requirements from error types.

[huonw]

Fixes rdar://problem/39548143.

[huonw]

@swift-ci please smoke test

[huonw]

Arguably it's fundamentally incorrect to have a NameAliasType with no SubstitutionMap, so maybe I should instead go fix that there (and/or hope #16268 helps).

[shahmishal]

@swift-ci please smoke test

[huonw]

Oh, nevermind, the problem is something different: the NameAliasType is theoretically behaving correctly here, as getSubstitutionListSize() for its generic signature returns 0, meaning the type doesn't store any substitutions and the map is empty. It does seem as if #16268 is very relevant to this misbehaviour.

[huonw]

It does seem as if #16268 is very relevant to this misbehaviour.

In fact, it fixed it.