Alternate proposal for origin tracking

[vil1]

Here's a proposal for tracking origin in IR nodes that:

• doesn't rely on maintaining a thread-local mutable variable (which is error prone as it is easy to forget to update said variable when needed)
• doesn't require any change in the structure of IR nodes

The proposed solution ensures that nodes return by the visit* methods of AST builders have their origin populated, by introducing a WithKnownOrigin marker trait.

Drawbacks are:

• addition of a mutable field in TreeNode, although said field is marked as private [this].
• the proposed change is fairly huge

Remaining work:

• finish implementing actual origin computation (by taking inspiration from #1199)
• remove CurrentOrigin
• propagate origin when relevant in the transform* methods

[github-actions[bot]]

Coverage tests results

448 tests  ±0   414 ✅ ±0   4s ⏱️ ±0s   6 suites ±0    34 💤 ±0    6 files   ±0     0 ❌ ±0 

Results for commit a5e3ce59. ± Comparison against base commit d4ad0cd2.