Rollup merge of #144741 - ShoyuVanilla:tilde-const-in-block, r=fee1-dead

fix: Error on illegal `[const]`s inside blocks within legal positions

Fixes rust-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:

https://github.com/rust-lang/rust/blob/3fb1b53a9dbfcdf37a4b67d35cde373316829930/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.

Author: samueltardieu