Rollup merge of #145233 - joshtriplett:cfg-select-expr, r=jieyouxu

cfg_select: Support unbraced expressions

Tracking issue for `cfg_select`: rust-lang/rust#115585

When operating on expressions, `cfg_select!` can now handle expressions
without braces. (It still requires braces for other things, such as
items.)

Expand the test coverage and documentation accordingly.

---

I'm not sure whether deciding to extend `cfg_select!` in this way is T-lang or T-libs-api. I've labeled for both, with the request that both teams don't block on each other. :)

Author: Kobzol