fix: Error on illegal `[const]`s inside blocks within legal positions
Fixesrust-lang/rust#132067
I initially considered moving `[const]` validations to `rustc_ast_lowering`, but that approach would require adding constness information to `AssocCtxt`, which introduces significant changes - especially within `rustc_expand` - just to support a single use case here:
3fb1b53a9d/compiler/rustc_ast_passes/src/ast_validation.rs (L1596-L1610)
Instead, I believe it's sufficient to simply "reset" `[const]` allowness whenever we enter a new block.
4cb0ebd24f