[clang] [C2y] Implement WG14 N3369 and N3469 (_Countof) (PR #133125)

[James Y Knight via cfe-commits]

jyknight wrote:

> We make a VariableArrayType whose element type is a VariableArrayType rather than a ConstantArrayType of size 7 whose element type is a VariableArrayType. 

Yea. Up till now I think there was no observable difference between the outer array having a constant or variable bound when the inner array is already a VLA, so it's perhaps not surprising it was modeled that way.

https://github.com/llvm/llvm-project/pull/133125