Unconditionally breaking `loop`s not allowed in constants.

[eddyb]

Technically a regression from 1.24 to 1.25, but I don't think anyone used this.
Example code which used to be allowed:

pub const FOO: () = loop { break; };

I'm pretty sure this changed with #47802, which added the FalseUnwind terminator to MIR, but didn't handle it in const-checking (which otherwise ignores cleanup blocks).

cc @matthewjasper @nikomatsakis

[Centril]

I think this is a technicality and that we can continue to let this be broken since no one said a peep about it since 1.25. The upside is that the syntactic surface of const eval is clearer.

[eddyb]

On the PR I explained why I think we should fix this even if we want to ban loops: #62274 (comment)