Fix effective context construction for NS_OPTIONS in linkage spec
When the ClangImporter imports a name, it associates it with a
an EffectiveClangContext. An EffectiveClangContext can be thought of
as the Clang scope the declaration will reside in, as far as importing
into Swift is concerned. This helps API notes and NS_SWIFT_NAME
to manipulate the SDK interface presented to Swift users.
When a entry is added to the Swift lookup table, it is associated
with a context. This context is a type and a name, used to effectively
namespace entries in the table. This context is derived from the
EffectiveClangContext associated with the name. This translation is
handled by SwiftLookupTable::translateContextDecl among other machinery.
This method in particular, understands only how to translate a set of
Clang nodes, and fails to create a context in other cases.
Prior to this patch, the EffectiveClangContext of declarations annotated
with UIKIT_EXTERN, with cxx-interop turned on, was a LinkageSpecDecl.
This results in context translation failure, and warnings produced in
SwiftLookupTable::finalizeLookupTable. This patch corrects name import
behavior to skip over the LinkageSpecDecl, and use the enclosing
TranslationUnit instead. This is appropriate and performed by
`determineEffectiveContext` as a LinkageSpecDecl is a so called
"transparent" context. That is its members are semantically declared and
accessible outside the context without additional qualification.
This patch tests using API notes, as that is the method UIKit uses. The
issue could just as easily be surface with a NS_SWIFT_NAME annotation.
Even without any annotation at all, the we would still fail to
translate, though there would likely be no corresponding warnings.
