enable c++11 mode in company-clang-arguments

[eklitzke]

This adds a new variable, c-c++-enable-c++11 which follows the existing c-c++ variable naming conventions.

When the value is not nil (it is nil by default) then company-clang-arguments are set up to use -std=c++11

[eklitzke]

I added another change like

(when c-c++-enable-c++11
      (setq flycheck-clang-language-standard "c++11"))

This should help people using flycheck and getting spelling errors about words like nullptr

[syl20bnr]

The variable is good but I would like the variable to be overridable at the project level via directory and file local variables.
So we can find some hook to be able to read the variable c-c++-enable-c++11 and set the flags accordingly.

[eklitzke]

Let me think about this idea some more and then update the diff.

[delaanthonio]

Why not have a c-c++-language-standard variable that flycheck can read from instead of c-c++-enable-c++11? This avoids needing a c-c++-enable-c++22 in the future.

[syl20bnr]

Thank you again (and still one year and a half late :-)) !

@beta1440 I'm refactoring the c-c++ layer, I'll take your remark into account.

Cherry-picked into develop branch, you can safely delete your branch.