block-expr: add new rule expr.block.result-value

[DanielEScherzer]

Document that the value of a block expression is based on the final operand (if present), just like the type.

[DanielEScherzer]

Discovered while working on a conference talk about rust, I couldn't find any rule saying that a block expression had a value equal to the final operand, so I added such a rule
Not quite sure how to phrase the part about diverging blocks - on the playground the following compiles with just some warnings about unreachable code:

fn main () {
    let result: String = { panic!("test") };
    let result: bool = { panic!("test") };
    let result: Vec<String> = { panic!("test") };
}

also, rule name could be improved, I originally was going to use expr.block.value but that is an existing rule

Blocks are always value expressions and evaluate the last operand in value expression context.

@rustbot label A-expressions

[DanielEScherzer]

From the review policy

If this would make a new guarantee about the language, this needs to go through the lang team to be accepted (unless the lang team has clearly accepted this guarantee elsewhere). Ask @traviscross if at all unsure about any of these.

Not sure if this is a new guarantee or just documenting a long-standing reality, @traviscross does this need to go through the lang team?

[traviscross]

We talked about this on the lang-docs call today. Revised as we discussed on the call. With that, look good to me. Thanks @DanielEScherzer for the PR.