Auto merge of #62075 - Centril:guardless-match-arms, r=petrochenkov

Remove `ast::Guard`

With the introduction of `ast::ExprKind::Let` in https://github.com/rust-lang/rust/pull/60861, the `ast::Guard` structure is now redundant in terms of representing [`if let` guards](https://github.com/rust-lang/rust/issues/51114) in AST since it can be represented by `ExprKind::Let` syntactically. Therefore, we remove `ast::Guard` here.

However, we keep `hir::Guard` because the semantic representation is a different matter and this story is more unclear right now (might involve `goto 'arm` in HIR or something...).

r? @petrochenkov

src/libsyntax/ast.rs

@@ -893,16 +893,11 @@ pub struct Local {
 pub struct Arm {
     pub attrs: Vec<Attribute>,
     pub pats: Vec<P<Pat>>,
-    pub guard: Option<Guard>,
+    pub guard: Option<P<Expr>>,
     pub body: P<Expr>,
     pub span: Span,
 }
 
-#[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
-pub enum Guard {
-    If(P<Expr>),
-}
-
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct Field {
     pub ident: Ident,