Prevent reads via original path of mutably borrowed content

[nikomatsakis]

RFC: https://gist.github.com/nikomatsakis/9272598

(Same as this blog post)

[pnkfelix]

Assigning 1.0, P-backcompat-lang.

[flaper87]

cc @flaper87

[zwarich]

I've got this implemented on top of my changes for #14318. It seems to be a simple change, modulo writing the new diagnostics. There are only 2 code changes required in the actual source base: https://gist.github.com/zwarich/13faf76055c960231ac9. There are more test changes required, but I haven't really looked into those.