[clang][cli] Store LangStandard::Kind in LangOptions

The `LangStandard::Kind` parsed from command line arguments is used to set up some `LangOption` defaults, but isn't stored anywhere.

To be able to generate `-std=` (in future patch), we need `CompilerInvocation` to not forget it.

This patch demonstrates another use-case: using `LangStd` to set up defaults of marshalled options.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D95342