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

[Hans Wennborg via llvm-commits]

================
@@ -226,35 +226,51 @@ enum class TagKind { Class, Struct, Union, Enum };
 
 enum class NodeKind {
   Unknown,
+
+  SymbolStart,
----------------
zmodem wrote:

I don't think we should introduce new enumerator values for the Start/End values. The guideline is that there should be one enumerator per concrete class.

How about something like:

```
enum class NodeKind {
  Unknown,

  SymbolStart,
  EncodedStringLiteral = SymbolStart,
  ...
  VariableSymbol,
  SymbolEnd = VariableSymbol,
  ...
```

https://github.com/llvm/llvm-project/pull/143410