Ban generic parameter direct shadowing in type & extension declaration contexts

[Douglas_Gregor]

@DevAndArtist

The only thing I‘m worried about that qualified lookup probably won‘t happen before Swift 5, and since it is kind of includes breaking changes I‘m not sue if we can ever get that feature in Swift post ABI stabilization.

I think @Douglas_Gregor could answer the question related to the addition of qualified lookup in post ABI stable Swift better.

Changes to name lookup affect the source language, but will have no ABI impact.

Doug