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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> Not really, using `LangOptions` directly instead of `std::unique_ptr<LangOptions>` allows reassignment too. I chose `std::unique_ptr` to:
> 
> * prevent increase of `ASTUnit` size,
> * avoid unconditional initialization of `ASTUnit::LangOpts` in the constructor (which would be an unnecessary change in this PR I think).

I think that is a reasonable choice.

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