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

[Ben Langmuir via llvm-commits]

benlangmuir wrote:

I haven't dug into this in depth yet, but did you consider keeping the normal function names for the `const` versions and only rename the mutating ones? My thinking is you could use `CowCompilerInvocation` as a drop-in replacement for `CompilerInvocation` in immutable settings.

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