Parse const block syntax

[dtolnay]

RFC: rust-lang/rfcs#2920
Tracking issue: rust-lang/rust#76001

As seen in the following test cases in rustc:

• https://github.com/rust-lang/rust/blob/7e9a36fa8a4ec06daec581e23f390389e05f25e4/src/test/ui/inline-const/const-expr-array-init.rs
• https://github.com/rust-lang/rust/blob/7e9a36fa8a4ec06daec581e23f390389e05f25e4/src/test/ui/inline-const/const-expr-basic.rs
• https://github.com/rust-lang/rust/blob/7e9a36fa8a4ec06daec581e23f390389e05f25e4/src/test/ui/inline-const/const-expr-reference.rs
• https://github.com/rust-lang/rust/blob/7e9a36fa8a4ec06daec581e23f390389e05f25e4/src/test/ui/inline-const/const-match-pat.rs
• https://github.com/rust-lang/rust/blob/7e9a36fa8a4ec06daec581e23f390389e05f25e4/src/test/ui/inline-const/const-match-pat-range.rs

[dtolnay]

Implemented in #930, #931, #932.