[Clang] [NFC] Introduce `ConstDynamicRecursiveASTVisitor` (#122991)

After some discussion around #116823, it was decided that it would be
nice to have a `const` variant of `DynamicRecursiveASTVisitor`, so this
pr does exactly that by making the main DRAV implementation a template
with a single `bool` template parameter that turns several function
parameters from a `T*` or `T&` to a `const T*` or `const T&`.

Since that made the implementation of a bunch of DRAV functions quite a
bit more verbose, I’ve moved most of them to be stamped out by a macro,
which imo makes it easier to understand what’s actually going on there.

For functions which already accepted `const` parameters in the original
RAV implementation, the parameter is `const` in both versions (e.g.
`TraverseTemplateArgument()` always takes a `const TemplateArgument&`);
conversely, parameters that are passed by value (e.g. in
`TraverseType()`, which takes a `QualType` by value) are *not* `const`
in either variant (i.e. the `QualType` argument is always just a
`QualType`, never a `const QualType`).

As a sanity check, I’ve also migrated some random visitor in the static
analyser to the `const` version (and indeed, it ends up simplifying the
code around that particular visitor actually). It would make sense to do
a pass over all visitors and change all which can be `const` use the
`const` version, but that can be done in a follow-up pr.

The [performance
impact](https://llvm-compile-time-tracker.com/compare.php?from=e3cd88a7be1dfd912bb6e7c7e888e7b442ffb5de&to=d55c5afe4a485b6d0431386e6f45cb44c1fc8883&stat=instructions:u)
of this change seems to be negligible. Clang’s binary size went up by
0.5%, but that’s expected considering that this effectively adds an
extra instantiation of `RecursiveASTVisitor`. Fortunately, this is of
course a one-time cost.