Unconditionally breaking `loop`s not allowed in constants.

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