Zir: move set_cold from Inst.Tag to Inst.Extended

If I could mark a builtin function as cold, I would mark @setCold as cold.
We have run out of `Zir.Inst.Tag`s so I had to move a tag from Zir.Inst.Tag to
Zir.Inst.Extended. This is because a new noreturn builtin will be added and
noreturn builtins cannot be part of Inst.Tag:
```
/// `noreturn` instructions may not go here; they must be part of the main `Tag` enum.
pub const Extended = enum(u16) {
```

Here's another reason I went for @setCold:
```
$ git grep setRuntimeSafety | wc -l
322
$ git grep setCold | wc -l
79
$ git grep setEvalBranchQuota | wc -l
82
```

This also simply removes @setCold from Autodoc and the docs frontend because
as far as I could understand it, builtins represented using Zir extended
instructions are not yet supported because I couldn't find
@setStackAlign or @setFloatMode there, either.

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