auto merge of #15650 : jakub-/rust/patterns-statics, r=pcwalton

This is accomplished by rewriting static expressions into equivalent patterns.
This way, patterns referencing static variables can both participate
in exhaustiveness analysis as well as be compiled down into the appropriate
branch of the decision trees that match expressions are codegened to.

Fixes #6533.
Fixes #13626.
Fixes #13731.
Fixes #14576.
Fixes #15393.

diff --cc src/librustc/middle/resolve.rs
Simple merge