[Support] Implement getMainExecutable on Solaris

[ro]

Summary

Many flang tests currently FAIL on Solaris because the module files aren't found. I could trace this to sys::fs::getMainExecutable not being implemented.

This patch does this and fixes all affected flang tests.

Tested on amd64-pc-solaris2.11.

[dim]

I can't vouch for the Solaris specific parts, but it looks sane to me. I guess you won't need to add any tests, since this will fix many now-failing tests? :)

llvm/lib/Support/Unix/Path.inc

 namespace llvm {
 namespace sys  {
 namespace fs {
 
 const file_t kInvalidFile = -1;
 
 #if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) ||     \
     defined(__minix) || defined(__FreeBSD_kernel__) || defined(__linux__) ||   \
-    defined(__CYGWIN__) || defined(__DragonFly__) || defined(_AIX) || defined(__GNU__)
+    defined(__CYGWIN__) || defined(__DragonFly__) || defined(_AIX) || defined(__GNU__) || \
+    (defined(__sun__) && defined(__svr4__))
 static int
 test_dir(char ret[PATH_MAX], const char *dir, const char *bin)
 {
   struct stat sb;
   char fullpath[PATH_MAX];
 
   int chars = snprintf(fullpath, PATH_MAX, "%s/%s", dir, bin);
   // We cannot write PATH_MAX characters because the string will be terminated
#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(__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)
+      return "";
+    return exe_path;
+  }
+  // Fall back to the classical detection.
+  if (getprogpath(exe_path, argv0) != NULL)
+    return exe_path;
 #elif defined(__MVS__)
   int token = 0;
   W_PSPROC buf;
   char exe_path[PS_PATHBLEN];
   pid_t pid = getpid();
 
   memset(&buf, 0, sizeof(buf));
   buf.ps_pathptr = exe_path;