When a tail expression isn't unit, we previously always suggested adding
a trailing `;` to turn it into a statement. This suggestion isn't
appropriate for any expression that doesn't have side-effects, as the
user will have likely wanted to call something else or do something with
the resulting value, instead of just discarding it.
Some newcomers are confused by the behavior of tail expressions,
interpreting that "leaving out the `;` makes it the return value".
To help them go in the right direction, suggest using `return` instead
when applicable.
This creates a new test directory, `ui/consts/control-flow` to hold
tests related to control flow in a const context. It also blesses all
existing tests with the new error messages, and adds new tests for the
`const_if_match` feature.
