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

This patch converts CXXNewExpr::InitializationStyle into a scoped enumat namespace scope. It also affirms the status quo by adding a new enumerator to represent implicit initializer.

This is a re-land of https://github.com/llvm/llvm-project/pull/71322 ace44893