rustc: Completely forbid borrows of unsafe statics

Summary:
It was possible to borrow unsafe static items in static initializers.
This patch implements a small `Visitor` that walks static initializer's
expressions and checks borrows aliasability.

Fixes #13005

Test Plan: make check

Differential Revision: http://phabricator.octayn.net/D2
auto merge of #13083 : FlaPer87/rust/issue-13005-borrow-unsafe-static, r=nikomatsakis

It was possible to borrow unsafe static items in static initializers.
This patch implements a small `Visitor` that walks static initializer's
expressions and checks borrows aliasability.

Fixes #13005

cc @nikomatsakis r?