higher level representation for initlist

[bcardosolopes]

It would be nice in the future to have a cir.initlist thing - a higher level representation for this (we even have a specialized AST node). Right now it would be really hard to recognize something came from an initializer list if we lower it this early. LoweringPrepare would then lower to what we currently get as of this PR.

Originally posted by @bcardosolopes in https://github.com/llvm/clangir/pull/764#pullrequestreview-2214210676