[clang] [llvm] Prefer std::getenv to ::getenv (PR #108529)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> If the implementation is conforming this doesn't do anything. In fact, they have to be the same function.

That was my understanding as well, thanks! So it sounds like the problem is that there's an underlying `::getenv` that isn't thread safe (which is conforming according to the C standard but not the latest POSIX standard)

C23 7.24.4.6p2: ... The getenv function is not required to avoid data races with other threads of execution that modify the environment list.

[POSIX](https://pubs.opengroup.org/onlinepubs/9799919799/): ... Some earlier versions of this standard did not require getenv() to be thread-safe because it was allowed to return a value pointing to an internal buffer. However, this behavior allowed by the ISO C standard is no longer allowed by POSIX.1. POSIX.1 requires the environment data to be available through environ[], so there is no reason why getenv() can't return a pointer to the actual data instead of a copy. Therefore getenv() is now required to be thread-safe (except when another thread modifies the environment). ... 

https://github.com/llvm/llvm-project/pull/108529