[PATCH] [WIP] Large refactoring of AST/APValue

[Dmitri Gribenko]

  I am not sure if shifting CheckConstantExpression into APValue is the correct layering, especially moving the diagnostic emission into it.

  In general, in LLVM and Clang we do assume a closed hierarchy of classes where appropriate, for example in the AST.  In that case, switching over the type discriminator is not a sign of bad design.

http://llvm-reviews.chandlerc.com/D3091