Auto merge of #144591 - RalfJung:pattern-valtrees, r=BoxyUwU

Patterns: represent constants as valtrees

Const patterns are always valtrees now. Let's represent that in the types. We use `ty::Value` for this since it nicely packages value and type, and has some convenient methods.

Cc `@Nadrieril` `@BoxyUwU`