kill rustc::cfg consumers

[nikomatsakis]

In general, rustc::cfg should be considered deprecated, and we ought to work to remove its consumers and rewrite them to operate on MIR:

• UnconditionalRecursion lint
• AST-based borrowck (that's what NLL is doing, of course)
• librustc_driver::pretty::print_flowgraph()

[phungleson]

Hey @nikomatsakis looks like some thing I can help with as rustc::cfg is relatively small.

Do you have any other suggestions for me to look further?