[clang] clang/limits.h: Avoid including limits.h twice (PR #120526)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > but that leads to the question of: why is including musl's limits.h a problem?
> 
> If we build `glibc` on `Alpine` with clang, we will meet the problem like
> 
> ```
> ./include/bits/xopen_lim.h:84:10: warning: "NL_NMAX" redefined
>    84 | # define NL_NMAX        INT_MAX
> ```
> 
> It's due to that
> 
> ```
>     glibc/include/limits.h  includes
>         /usr/lib/llvm19/lib/clang/19/include/limits.h  includes
>             /usr/include/limits.h
> ```

Yes and no. It's due to the redefinitions, for sure. But that diagnostic is suppressed in system headers: https://godbolt.org/z/Mb7Kh975f, so I think we need to understand why there's a `limits.h` being included as though it were not a system header, because that's a bug.

https://github.com/llvm/llvm-project/pull/120526