[MsDemangle] Use LLVM style RTTI for AST nodes (#143410)

The inheritance hierarchy for `llvm::ms_demangle::Node`
([doxygen](https://llvm.org/doxygen/structllvm_1_1ms__demangle_1_1Node.html))
is a bit more involved. One thing that's missing without RTTI is the
ability to determine if a node is a symbol, identifier, or type (or one
would need to check for every kind).

This PR adds support for `dyn_cast`, `isa`, and friends to
`llvm::ms_demangle::Node`. As the type already has a `kind()`, this
mainly adds `classof` to the nodes as well as some start and end markers
in the `NodeKind` enum.

Author: Nerixyz