[clang] [clang] Hide the `LangOptions` pointer from `CompilerInvocation` (PR #137675)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> The `unique_ptr` in `ASTUnit` always stores a **copy** of the `shared_ptr` in `CompilerInvocation`.

And we need `unique_ptr` in `ASTUnit` instead of a direct value so we can change LangOpts in `ASTUnit::Parse`?

https://github.com/llvm/llvm-project/pull/137675