[clang][cli] NFC: Promote ParseLangArgs and ParseCodeGenArgs to members

This patch promotes `ParseLangArgs` and `ParseCodeGenArgs` to members of `CompilerInvocation`. That will be useful in the following patch D94682, where we need to access protected members of `LangOptions` and `CodeGenOptions`. Both of those classes already have `friend CompilerInvocation`.

This is cleaner than keeping those functions freestanding and having to specify the exact signature of both in extra `friend` declarations.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D94681