[clang] [clang][Driver] Include stdc-predef.h by default on musl (PR #202919)

[Ross Burton via cfe-commits]

rossburton wrote:

I tested this patch but it fails to build musl itself (and other freestanding components before musl has been built), as `stdc-predef.h` doesn't exist yet.

https://github.com/llvm/llvm-project/pull/202919