[clang] [clang-tools-extra] [flang] [llvm] [flang] add SYSTEM runtime and lowering intrinsics support (PR #74309)

[Yi Wu via llvm-commits]

yi-wu-arm wrote:

Hi @jeanPerier sorry to throw a pin, do you mind doing a code review on this pr? This patch calls `std::system` to implement the `SYSTEM` gnu extension. The command passing will be ensured null-terminated, it is achieved by creating a length+1 char using malloc and free if inputted char does not have null terminator. Ther is a default integer optional para `exitstat` to capture the return value of `std::system`. Thanks in advance.
https://gcc.gnu.org/onlinedocs/gfortran/SYSTEM.html

https://github.com/llvm/llvm-project/pull/74309