It turns out that the substitution for expression comparing also needs
an unevaluated context, otherwise any reference to immediate functions
might not be properly handled.
As a fallout, this also guards the VLA transformation under unevaluated
context
with `InConditionallyConstantEvaluateContext` to avoid duplicate
diagnostics.
Fixes https://github.com/llvm/llvm-project/issues/123472
---------
Co-authored-by: cor3ntin <corentinjabot@gmail.com>
69d0c4c167
