Address Issue 14270 by making `cmt::freely_aliasable` result more fine-grained.

Address Issue 14270 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-aliasble (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.