[clang] [clang] Fixup last value in DarwinPlatformKind enum (PR #81011)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> > Why didn't this trigger:
> > https://github.com/llvm/llvm-project/blob/ab92f6274b7c3a4b4445d47017bc481aa919545f/clang/lib/Driver/ToolChains/Darwin.cpp#L1906
> > 
> > ?
> 
> XROS is missing in the env vars too:
> 
> https://github.com/llvm/llvm-project/blob/ab92f6274b7c3a4b4445d47017bc481aa919545f/clang/lib/Driver/ToolChains/Darwin.cpp#L1899

Yes. Adding proper `XROS_DEPLOYMENT_TARGET` support is a bigger patch, I'll need to update this PR to reflect this. 

https://github.com/llvm/llvm-project/pull/81011