swift234

[InterfaceGen] Fix deserializing submodule ImportDecls

Use the isScoped() bit to distinguish scoped imports from submodule
imports (both of which are split by null bytes in the string) so that we
don't try to lookup a submodule name as if it were a decl (leading to
assertion failures when it wasn't found).

This fixes interface generation of swift modules that import clang
submodules.

rdar://problem/24534122