For some reason, findDeclContext() finds the special ClangImporter
module from inside lldb, and then we were skipping the logic in
findForeignTypeDecl(). So check for the foreign type case first.
MetadataLookup gives special treatment to imported Objective-C classes,
since there's no nominal type descriptor and metadata is obtained
directly by calling into the Objective-C runtime.
Remote reflection also gives special treatment to imported Objective-C
classes; they don't have field descriptors.
However, the ASTDemangler needs to treat them like ordinary classes,
in particular it wants to preserve the generic arguments here so that
we can round-trip debug info.
Translate the metadata for the generic requirements of an extension context
into a demangle tree that is associated with the demangling of an extension.
Teach the ASTDemangler how to handle class layout constraints as well.
With this, RemoteAST can resolve types nested within most constrained
extensions.
Debug info uses a special mangling where type aliases can be
represented without being desugared; attempt to reconstruct
the TypeAliasType in this case.
It's clever how it leverages the type checker to check generic arguments,
but calling checkGenericArguments() would have been simpler, and it doesn't
work for interface types anyway, so we would fail to demangle those.
Let's just cut this all out for now and build a BoundGenericType directly.
When an enum is imported as an error, the imported type itself becomes
a nested type 'Code' of its wrapper type, so you get a type like
'MyError.Code'. However in the mangling grammar the type is a
child of a module and not another type.
This was tripping up TypeDecoder, which would check parent types for
validity and throw out the demangling since it looked invalid.
However since this case really is valid, just skip the whole parent
type mess when the type is imported and non-generic.
