[clang] clang: Remove unnecessary cstdlib include (PR #209707)

[Ruoyu Zhong via cfe-commits]

https://github.com/ZhongRuoyu commented:

> but this file does not directly use getenv

I think it *is* used:

https://github.com/llvm/llvm-project/blob/f48b09bbe0654fcd7ddf6b8a8aec807d8552a816/clang/lib/Driver/Driver.cpp#L1143

https://github.com/llvm/llvm-project/blob/f48b09bbe0654fcd7ddf6b8a8aec807d8552a816/clang/lib/Driver/Driver.cpp#L1363


https://github.com/llvm/llvm-project/pull/209707