get rid of `RefCell` in `TransitiveRelation`
This is one of the jobs in `Pending refactorings` in #48685. The parallel-compiler's work has been suspended for quite some time, but I think I can pick it up gradually. I think this PR should be a start.
Regarding the refactoring of `TransitiveRelation`, `@nikomatsakis` has proposed [two(three?) schemes](https://github.com/rust-lang/rust/pull/48587#issuecomment-369336651). In order to satisfy both compilation efficiency and robustness, I think adding the `freeze` method may be the best solution, although it requires relatively more code changes.
a8a33cf271