[ClangImporter] Fix IUO ordering bug
The IUO-ness of imported declarations is not actually computed by IsImplicitlyUnwrappedOptionalRequest. Instead, ClangImporter manually sets the bit to `true` after the declaration’s type is imported and expects IsImplicitlyUnwrappedOptionalRequest to always set it to `false` for all other imported declarations.
Normally, declaration types are imported greedily as soon as the declaration is created. However, a ClangImporter refactoring in apple/swift#61026 deferred the import of a VarDecl’s type, and therefore the setting of its IUO bit, until the first time InterfaceTypeRequest is evaluated.
It turns out that there is nothing to guarantee that InterfaceTypeRequest will be evaluated before IsImplicitlyUnwrappedOptionalRequest, so if isImplicitlyUnwrappedOptional() was fetched before getInterfaceType() was called, it would return an incorrect result. The only known client that accesses the information in this order is the API digester, but in theory any part of the compiler could fall into this trap.
Force the evaluation of InterfaceTypeRequest during IsImplicitlyUnwrappedOptionalRequest when necessary to compute the IUO bit for an imported VarDecl, and add a test to prove that this fixes the observed bug in the API digester.
imported_iuo.h