Fix grammar for RangePatternBound regarding literals

[traviscross]

The compiler, in parsing, accepts many more kinds of literals in the bound of a range pattern than what we had documented, and as with PatternWithoutRange, the parser allows each of these literals to be prefixed with -. E.g.

#[cfg(any())]
match () {
    ..-true | ..-false => (),
    ..-'x' => (),
    ..-b'x' => (),
    ..-"x" => (),
    ..-r"x" => (),
    ..-br"x" => (),
    ..-c"x" => (),
    ..-cr"x" => (),
    ..-1 => (),
    ..-1.1 => (),
}

Let's fix this by adjusting the RangePatternBound production to use LiteralPattern. In a separate PR, we've adjusted LiteralPattern to allow for a minus sign ahead of all literal expressions.

To help the reader, let's also add a note that highlights that what we discuss later as being allowed semantically is more restrictive than what we allow in parsing (and therefore in the grammar).

cc @ehuss

[ehuss]

Thanks!