Remove MacDelimiter.

It's the same as `Delimiter`, minus the `Invisible` variant. I'm
generally in favour of using types to make impossible states
unrepresentable, but this one feels very low-value, and the conversions
between the two types are annoying and confusing.

Look at the change in `src/tools/rustfmt/src/expr.rs` for an example:
the old code converted from `MacDelimiter` to `Delimiter` and back
again, for no good reason. This suggests the author was confused about
the types.

compiler/rustc_ast/src/ast.rs

@@ -14,7 +14,7 @@
 //! - [`Generics`], [`GenericParam`], [`WhereClause`]: Metadata associated with generic parameters.
 //! - [`EnumDef`] and [`Variant`]: Enum declaration.
 //! - [`MetaItemLit`] and [`LitKind`]: Literal expressions.
-//! - [`MacroDef`], [`MacStmtStyle`], [`MacCall`], [`MacDelimiter`]: Macro definition and invocation.
+//! - [`MacroDef`], [`MacStmtStyle`], [`MacCall`]: Macro definition and invocation.
 //! - [`Attribute`]: Metadata associated with item.
 //! - [`UnOp`], [`BinOp`], and [`BinOpKind`]: Unary and binary operators.
 
@@ -1693,7 +1693,7 @@ where
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct DelimArgs {
     pub dspan: DelimSpan,
-    pub delim: MacDelimiter,
+    pub delim: Delimiter, // Note: `Delimiter::Invisible` never occurs
     pub tokens: TokenStream,
 }
 
@@ -1701,7 +1701,7 @@ impl DelimArgs {
     /// Whether a macro with these arguments needs a semicolon
     /// when used as a standalone item or statement.
     pub fn need_semicolon(&self) -> bool {
-        !matches!(self, DelimArgs { delim: MacDelimiter::Brace, .. })
+        !matches!(self, DelimArgs { delim: Delimiter::Brace, .. })
     }
 }
 
@@ -1717,32 +1717,6 @@ where
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug, HashStable_Generic)]
-pub enum MacDelimiter {
-    Parenthesis,
-    Bracket,
-    Brace,
-}
-
-impl MacDelimiter {
-    pub fn to_token(self) -> Delimiter {
-        match self {
-            MacDelimiter::Parenthesis => Delimiter::Parenthesis,
-            MacDelimiter::Bracket => Delimiter::Bracket,
-            MacDelimiter::Brace => Delimiter::Brace,
-        }
-    }
-
-    pub fn from_token(delim: Delimiter) -> Option<MacDelimiter> {
-        match delim {
-            Delimiter::Parenthesis => Some(MacDelimiter::Parenthesis),
-            Delimiter::Bracket => Some(MacDelimiter::Bracket),
-            Delimiter::Brace => Some(MacDelimiter::Brace),
-            Delimiter::Invisible => None,
-        }
-    }
-}
-
 /// Represents a macro definition.
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct MacroDef {