Support early const eval for inline `const{}` expressions

[BergmannAtmet]

Related:
https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/const.20binding.20vs.2E.20const.7B.7D.20evaluation/near/558550788

Const eval is not guaranteed to be done at check time, especially with const{} expressions.

For example:

// check error
const X: NonZeroU32 = NonZeroU32::new(0).unwrap();
// check success, build error
let x: NonZeroU32 = const{NonZeroU32::new(0).unwrap()};

rust-analyzer catching such errors would be a great addition.

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I hope this is not a duplicate, since it's not easy to search for this.

[Veykril]

Not really a dupe but our const eval is very bare bones so we don't catch a lot of things in the first place. We also don't evaluate constants in a bunch of contexts yet (this one included)