[Support] Implement getMainExecutable for OpenBSD

[brad]

Summary

This uses just argv[0] on OpenBSD. OpenBSD does not support the /proc filesystem.

[MaskRay]

You may want to do more testing and study some changes in this function.
getMainExecutable is used by clang driver and libclang (C API) to find resource directories.

llvm/lib/Support/Unix/Path.inc

if (aux->a_type == AT_EXECPATH) {
       if (realpath((char *)aux->a_un.a_ptr, link_path))
         return link_path;
     }
   }
 #endif
   // Fall back to argv[0] if auxiliary vectors are not available.
   if (getprogpath(exe_path, argv0) != NULL)
     return exe_path;
-#elif defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__) || \
-    defined(__FreeBSD_kernel__) || defined(_AIX)
+#elif defined(_AIX) || defined(__DragonFly__) || defined(__FreeBSD_kernel__) || \
+    defined(__NetBSD__)
   const char *curproc = "/proc/curproc/file";
   char exe_path[PATH_MAX];
   if (sys::fs::exists(curproc)) {
     ssize_t len = readlink(curproc, exe_path, sizeof(exe_path));
     if (len > 0) {
       // Null terminate the string for realpath. readlink never null
       // terminates its output.
       len = std::min(len, ssize_t(sizeof(exe_path) - 1));
#else
     char real_path[PATH_MAX];
     if (realpath(exe_path, real_path))
       return std::string(real_path);
 #endif
   }
   // Fall back to the classical detection.
   if (getprogpath(exe_path, argv0))
     return exe_path;
+#elif defined(__OpenBSD__)
+  char exe_path[PATH_MAX];
+  // argv[0] only
+  if (getprogpath(exe_path, argv0) != NULL)
+    return exe_path;
 #elif defined(__sun__) && defined(__svr4__)
   char exe_path[PATH_MAX];
   const char *aPath = "/proc/self/execname";
   if (sys::fs::exists(aPath)) {
     int fd = open(aPath, O_RDONLY);
     if (fd == -1)
       return "";
     if (read(fd, exe_path, sizeof(exe_path)) < 0)
if (auto EC = fs::status(Path, s, FollowSymlinks))
     return EC;
   return s;
 }
 
 //
 // FreeBSD optionally provides /proc/self/fd, but it is incompatible with
 // Linux. The thing to use is realpath.
 //
-#if !defined(__FreeBSD__)
+#if !defined(__FreeBSD__) && !defined(__OpenBSD__)
 #define TRY_PROC_SELF_FD
 #endif
 
 #if !defined(F_GETPATH) && defined(TRY_PROC_SELF_FD)
 static bool hasProcSelfFD() {
   // If we have a /proc filesystem mounted, we can quickly establish the
   // real name of the file with readlink
   static const bool Result = (::access("/proc/self/fd", R_OK) == 0);