[llvm] [opt] Move InitLLVM ownership to the standalone entry point (PR #222559)

[Anutosh Bhat via llvm-commits]

anutosh491 wrote:

Hey @wsmoses, since you introduced LLVMOptDriver as a reusable library in #79205, would you like to help me with the reviews here !?

Also tagging @aganea for the broader in-process tooling/session design

https://github.com/llvm/llvm-project/pull/222559