Fix grammar for LiteralPattern regarding -

We had documented that only numeric literals in patterns can be
prefixed by `-` (minus), but the Rust parser happily accepts a minus
ahead of all literals in patterns.  E.g.:

```rust
match () {
    -true | -false => (),
    -'x' => (),
    -b'x' => (),
    -"x" => (),
    -r"x" => (),
    -br"x" => (),
    -c"x" => (),
    -cr"x" => (),
    -1 => (),
    -1.1 => (),
}
```

In the compiler, this happens in `Parser::parse_literal_maybe_minus`
and `Token::can_begin_literal_maybe_minus`.

Let's fix this by defining `LiteralPattern` as a `LiteralExpression`
optionally prefixed by the minus sign.  This better matches how the
`rustc` AST models this.

Author: traviscross

src/patterns.md

@@ -139,23 +139,11 @@ r[patterns.literal]
 
 r[patterns.literal.syntax]
 ```grammar,patterns
-LiteralPattern ->
-      `true` | `false`
-    | CHAR_LITERAL
-    | BYTE_LITERAL
-    | STRING_LITERAL
-    | RAW_STRING_LITERAL
-    | BYTE_STRING_LITERAL
-    | RAW_BYTE_STRING_LITERAL
-    | C_STRING_LITERAL
-    | RAW_C_STRING_LITERAL
-    | `-`? INTEGER_LITERAL
-    | `-`? FLOAT_LITERAL
+LiteralPattern -> `-`? LiteralExpression
 ```
 
 r[patterns.literal.intro]
-_Literal patterns_ match exactly the same value as what is created by the literal.
-Since negative numbers are not [literals], literal patterns also accept an optional minus sign before the literal, which acts like the negation operator.
+_Literal patterns_ match exactly the same value as what is created by the literal. Since negative numbers are not [literals], literals in patterns may be prefixed by an optional minus sign, which acts like the negation operator.
 
 > [!WARNING]
 > C string and raw C string literals are accepted in literal patterns, but `&CStr` doesn't implement structural equality (`#[derive(Eq, PartialEq)]`) and therefore any such `match` on a `&CStr` will be rejected with a type error.