[libc-commits] [libc] [libc] Move __LLVM_LIBC__ define to __llvm-libc-common.h (PR #126877)

[Petr Hosek via libc-commits]

petrhosek wrote:

> I'm okay with this, but including `features.h` and nothing else still needs to define this.

It will since [`features.h` includes `__llvm-libc-common.h`](https://github.com/llvm/llvm-project/blob/a3e2075e5008cefc2e896f1558b46f0882321ff5/libc/include/features.h.def#L12).

https://github.com/llvm/llvm-project/pull/126877