RecursiveASTVisitor was recursing into the subtrees of an old root if it
was changed in on_entry callback. Fix that by querying root pointer just
after on_entry callback returns. While on it, also use
`AK::TemporaryChange` instead of setting `m_current_subtree_pointer`
manually.
As it turns out, `FunctionCallCanonicalizationPass` was relying on being
able to replace tree on entry, and the bug in RecursiveASTVisitor made
the pass to not fully canonicalize nested function calls.
The changes to GenericASTPass.cpp alone are enough to fix the problem
but it is canonical (for some definition of canonicity) to only change
trees in on_leave. Therefore, the commit also switches
FunctionCallCanonicalizationPass to on_leave callback.
A test for this fix and one from the previous commit is also included.
This class stores a non-owning raw pointer to a member of `Node`, so
extra care is needed to ensure that referenced `Node`s will be alive
by the time `NodeSubtreePointer` is used. Since we only need to use this
class while traversing AST in `RecursiveASTVisitor`, access to class
methods can be restricted using `Badge<RecursiveASTVisitor>`.
