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 removing the `-`s from within `LiteralPattern` and
instead allowing `LiteralPattern` to be prefixed by an optional `-`.
This better matches how the `rustc` AST models this as compared with
the alternative of including the optional `-`s pervasively in the
`LiteralPattern` production.

Author: traviscross

src/patterns.md

@@ -10,7 +10,7 @@ PatternNoTopAlt ->
     | RangePattern
 
 PatternWithoutRange ->
-      LiteralPattern
+      `-`? LiteralPattern
     | IdentifierPattern
     | WildcardPattern
     | RestPattern
@@ -149,8 +149,8 @@ LiteralPattern ->
     | RAW_BYTE_STRING_LITERAL
     | C_STRING_LITERAL
     | RAW_C_STRING_LITERAL
-    | `-`? INTEGER_LITERAL
-    | `-`? FLOAT_LITERAL
+    | INTEGER_LITERAL
+    | FLOAT_LITERAL
 ```
 
 r[patterns.literal.intro]