Auto merge of #148182 - saethlin:trivial-consts-recursive, r=eholk

Accept trivial consts based on trivial consts

This is an expansion of rust-lang/rust#148040.

The previous implementation only accepted trivial consts that assign a literal. For example:
```rust
const A: usize = 0;
const B: usize = A;
```
Before this PR, only `A` was a trivial const. Now `B` is too.

Author: bors