[llvm] [clang][deps] Optimize command line generation (PR #65412)

[Ben Langmuir via llvm-commits]

benlangmuir wrote:

> There are places in Clang where we store the result of the const function in some kind of long-lived data structure, and someplace else modify the result of the non-const function, expecting the change to be reflected.

I would be comfortable with this, because you would still have to call a distinctly named function (getMut...) in order to do the mutation itself IIUC.  If they are const-casting of course that wouldn't be enough.

> Note that you can use CompilerInvocationBase to get the behavior you're describing.

I think `CompilerInvocationBase` goes a step farther than what I described, because it would give you the mutating methods with the original names, no?

https://github.com/llvm/llvm-project/pull/65412