Auto merge of #139083 - petrochenkov:ctxtdecod3, r=nnethercote

hygiene: Rewrite `apply_mark_internal` to be more understandable

The previous implementation allocated new `SyntaxContext`s in the inverted order, and it was generally very hard to understand why its result matches what the `opaque` and `opaque_and_semitransparent` field docs promise.
```rust
/// This context, but with all transparent and semi-transparent expansions filtered away.
opaque: SyntaxContext,
/// This context, but with all transparent expansions filtered away.
opaque_and_semitransparent: SyntaxContext,
```
It also couldn't be easily reused for the case where the context id is pre-reserved like in #129827.

The new implementation tries to follow the docs in a more straightforward way.
I did the transformation in small steps, so it indeed matches the old implementation, not just the docs.
So I suggest reading only the new version.

Author: bors