SE-0382: Expression Macros

[s-k (Stephen Kockentiedt)]

@Douglas_Gregor

I (very recently) fixed a compiler bug that caused the incorrect error messages, and with my locally-built compiler (and adding a bunch of @expression utterances), your result-builder macro works. My fix hasn't made it into a snapshot yet, but here's a one-off toolchain that contains it.

Thank you! I will try to make time to work on it some more and give feedback on what I learned.

@tera

Cool stuff. Assuming function returning a value can this somehow print:

    enter: doSomething(42, b: 256)
    exit:  doSomething(42, b: 256) -> 53

where 53 is the function result?

I think this would need a declaration macro that must be applied to the function itself.