Issue Description:

See https://forums.swift.org/t/too-complex-closure-return-type/49897 – it's not clear why a closure is "too complex" because from a user perspective, it looks quite simple.

In case there are multiple statements, we can provide a more specialized diagnostic saying that the compiler doesn't support inferring types of closures with multiple statements.

_ = { // error: Unable to infer complex closure return type; add explicit type to disambiguate
    _ = "" // or anything else here
    return ""
}