Immutable statics should only allow types which are Freeze

[alexcrichton]

All statics are placed in rodata by LLVM (we mark them as constant), so any attempt at mutation will not succeed (it will segfault).

See #11583 (comment) and #11583 (comment). I wanted to make an official issue out of those comments.

Despite the discussion in #10577 to work on values of immutable statics, I think that this Freeze bound should apply to the type of the static instead.

Nominating.

[emberian]

Makes sense, kinda weird that we aren't doing it already.

[flaper87]

Makes sense to me as well! Working on it!