Rework pattern and expression nonterminal kinds.

Merge `PatParam`/`PatWithOr`, and `Expr`/`Expr2021`, for a few reasons.

- It's conceptually nice, because the two pattern kinds and the two
  expression kinds are very similar.

- With expressions in particular, there are several places where both
  expression kinds get the same treatment.

- It removes one unreachable match arm.

- Most importantly, for #124141 I will need to introduce a new type
  `MetaVarKind` that is very similar to `NonterminalKind`, but records a
  couple of extra fields for expression metavars. It's nicer to have a
  single `MetaVarKind::Expr` expression variant to hold those extra
  fields instead of duplicating them across two variants
  `MetaVarKind::{Expr,Expr2021}`. And then it makes sense for patterns
  to be treated the same way, and for `NonterminalKind` to also be
  treated the same way.

I also clarified the comments, because I have long found them a little
hard to understand.

src/parse/macros/mod.rs

@@ -1,4 +1,4 @@
-use rustc_ast::token::{Delimiter, NonterminalKind, TokenKind};
+use rustc_ast::token::{Delimiter, NonterminalKind, NtExprKind::*, NtPatKind::*, TokenKind};
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::{ast, ptr};
 use rustc_parse::parser::{ForceCollect, Parser, Recovery};
@@ -48,7 +48,7 @@ macro_rules! parse_macro_arg {
 
     parse_macro_arg!(
         Expr,
-        NonterminalKind::Expr,
+        NonterminalKind::Expr(Expr),
         |parser: &mut Parser<'b>| parser.parse_expr(),
         |x: ptr::P<ast::Expr>| Some(x)
     );
@@ -60,7 +60,7 @@ macro_rules! parse_macro_arg {
     );
     parse_macro_arg!(
         Pat,
-        NonterminalKind::PatParam { inferred: false },
+        NonterminalKind::Pat(PatParam { inferred: false }),
         |parser: &mut Parser<'b>| parser.parse_pat_no_top_alt(None, None),
         |x: ptr::P<ast::Pat>| Some(x)
     );