swift234

[ClangImporter] Handle submodule imports.

Specifically, handle them by also importing the top-level module. This is
unfortunate, but at least lets people /access/ things in explicit submodules,
even if it doesn't let them limit their import to a specific submodule.

    (swift) import OpenGL.GL3
    (swift) glGetString
    // r0 : (GLenum) -> ConstUnsafePointer<GLubyte> = (Function)
    (swift) OpenGL.glGetString
    // r1 : (GLenum) -> ConstUnsafePointer<GLubyte> = (Function)

One unfortunate side effect of having a single Clang ASTContext is that if
one Swift module imports a Clang submodule, every Swift module can now see
it. That means /mixing/ incompatible submodules, such as OpenGL.GL and
OpenGL.GL3, still won't work. Filed <rdar://problem/17756745> for that.

<rdar://problem/13140302>

Swift SVN r20288