[flang] [clang-tools-extra] [clang] [llvm] [flang] GETLOG runtime and extension implementation: get login username (PR #74628)

[Yi Wu via cfe-commits]

yi-wu-arm wrote:

`getlog` library intrinsic will first retrieve user login name using `getlog_r` on Linux and `GetUserName` on Windows. If that returns an error, it will then use environment variable to get username.

https://github.com/llvm/llvm-project/pull/74628