There can be Macros that are tagged with `modifiable`. Thus verifying
`canModifyAllDescendants` is not sufficient to avoid macros when deep
copying.
We think the `TokenBuffer` could inform us whether a `Token` comes from
a macro. We'll look into that when we can surface this information
easily, for instance in unit tests for `ComputeReplacements`.
Differential Revision: https://reviews.llvm.org/D88034
In a future patch
* Implement helper function to generate Trees for tests
* and test Tree methods, namely `findFirstLeaf` and `findLastLeaf`
Differential Revision: https://reviews.llvm.org/D87533