Issue Description:

Given a Clang module Mod and SubMod, where SubMod is a submodule of Mod, and has the following declaration:

extern void fromSubmodule();

When compiling a same-named Swift module Mod that has a scoped import for Mod.SubMod.fromSubmodule, for example:

import func Mixed.Submodule.fromSubmodule

We incorrectly emit an error:

error: function 'fromSubmodule' does not exist in module 'Mod'
import func Mod.SubMod.fromSubmodule
            ^

However we should really allow this to compile.