New syntax in rustfmt 1.4.0

[topecongiro]

There have been/will be quite a few updates to the Rust syntax. This PR keeps track of those updates so that I don't get panic on updating rustc-ap-* the next time.

Major syntax updates

• Remove IfLet and WhileLet, and introducing Let to ExprKind
• Remove Guard
• Remove await!() macro call
• Add Rest to PatKind
• Add attributes on function parameters (Support for attributes in function parameters #3623)

Related PRs

• [let_chains, 2/6] Introduce Let(..) in AST, remove IfLet + WhileLet and parse let chains rust#60861
• Remove ast::Guard rust#62075
• Implement RFC 2707 + Parser recovery for range patterns rust#62550