Add type-check test coverage for `ExprData::True`

[nikomatsakis]

Parent tracking issue: #265

The ExprData::True variant has borrow-check coverage (e.g., min_problem_case_3) but no type-checking coverage in mir_typeck.rs.

Add a test that uses the true literal. This could be combined with a Stmt::If test. See the parent tracking issue for instructions on how to add tests.

[xonx4l]

I have a solution for this.

[xonx4l]

@rustbot claim

[tiif]

This issue is completed.