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

[Volodymyr Sapsai via cfe-commits]

https://github.com/vsapsai commented:

Also looks like [this assert](https://github.com/llvm/llvm-project/blob/14469a8726fc1d038f41c7e3e29a84ec8f52d013/clang/include/clang/Frontend/ASTUnit.h#L472) doesn't serve its value anymore
```c++
  const LangOptions &getLangOpts() const {
    assert(LangOpts && "ASTUnit does not have language options");
    return *LangOpts;
  }
```

But it is a minor cleanup, not blocking.

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