[clang] Add back include for AutoConvert.h as it's needed for z/OS (PR #135430)

[Sean Perry via cfe-commits]

perry-ca wrote:

> I just happened to look into the implementation of AutoConvert.h, and I see the entire interface is defined away when not targeting zOS. This means that all call sites need to be conditional on `ifdef __MVS__`, which means we have ugly ifdef droppings all over the codebase, which harms readability and understandability.
> 
> Can whoever owns this redesign the header so that it provides inline functions which have default implementations that return false / no_error to disable their functionality on non-zOS platforms? Otherwise this failure mode will happen again in the future. This seems like feedback that should've come up in the original review (https://reviews.llvm.org/D100483), but I don't recognize enough of the contributor names there to confirm that there was some approval from maintainers or the broader community, it looks like this was mostly approved by zOS stakeholders.

I had the same reflection.  I'd get that change made.   Thanks

https://github.com/llvm/llvm-project/pull/135430