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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

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

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