Remove usage of `do` keyword, in line with #10815.

I haven't removed the keyword yet; this is just a matter of stopping
using it. Changing the `do` keyword to complaining of obsolete syntax is
the next step.

Also I have *not* removed the `do` keyword tests; they still use `do`.
These are:

- src/test/compile-fail/do-lambda-requires-braces.rs
- src/test/compile-fail/do1.rs
- src/test/compile-fail/do2.rs
- src/test/compile-fail/issue-3044.rs
- src/test/run-pass/block-arg-can-be-followed-by-binop.rs
- src/test/run-pass/block-arg-can-be-followed-by-block-arg.rs
- src/test/run-pass/block-arg-can-be-followed-by-call.rs
- src/test/run-pass/block-arg-in-parentheses.rs
- src/test/run-pass/block-arg-used-as-any.rs
- src/test/run-pass/block-arg.rs
- src/test/run-pass/do-empty-args.rs
- src/test/run-pass/do-no-args.rs
- src/test/run-pass/do1.rs
- src/test/run-pass/do2.rs
- src/test/run-pass/do3.rs