[clang] [clang-tools-extra] [clang][NFC] Refactor `CXXNewExpr::InitializationStyle` (re-land) (PR #71417)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> I agree this stretches the definition of NFC commit.
But it was dully reviewed and approved https://github.com/llvm/llvm-project/pull/71322

I agree with this assessment. I think it really started as regular NFC, but then me and Aaron realized that we can get rid of some ugly code if we properly model implicit initialization.

https://github.com/llvm/llvm-project/pull/71417