Rollup merge of #23859 - pnkfelix:fsk-lesser-box, r=nikomatsakis

Disallow writing through mutable pointers stored in non-mut Box.

Fix #14270 

The fix works by making `cmt::freely_aliasable` result more fine-grained.

Instead of encoding the aliasability (i.e. whether the cmt is uniquely writable or not) as an option, now pass back an enum indicating either: 1. freely-aliasable (thus not uniquely-writable), 2. non-aliasable (thus uniquely writable), or 3. unique but immutable (and thus not uniquely writable, according to proposal from issue #14270.)

This is all of course a giant hack that will hopefully go away with an eventually removal of special treatment of `Box<T>` (aka `ty_unique`) from the compiler.

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