getMainExecutable: handle realpath() failure, falling back to getprogpath().

[sammccall]

Summary

Previously, we'd pass a nullptr to std::string and crash().

This case happens when the binary is deleted while being used (e.g. rebuilding clangd).

[kadircet]

LGTM, thanks!

llvm/trunk/lib/Support/Unix/Path.inc

if (sys::fs::exists(aPath)) {
     len = std::min(len, ssize_t(sizeof(exe_path) - 1));
     exe_path[len] = '\0';
 
     // On Linux, /proc/self/exe always looks through symlinks. However, on
     // GNU/Hurd, /proc/self/exe is a symlink to the path that was used to start
     // the program, and not the eventual binary file. Therefore, call realpath
     // so this behaves the same on all platforms.
 #if _POSIX_VERSION >= 200112 || defined(__GLIBC__)
-    char *real_path = realpath(exe_path, NULL);
+    if (char *real_path = realpath(exe_path, NULL)) {
       std::string ret = std::string(real_path);
       free(real_path);
       return ret;
+    }
 #else
     char real_path[MAXPATHLEN];
-    realpath(exe_path, real_path);
+    if (realpath(exe_path, real_path))
       return std::string(real_path);
 #endif
-  } else {
-    // Fall back to the classical detection.
-    if (getprogpath(exe_path, argv0))
-      return exe_path;
-  }
+  }
 #elif defined(HAVE_DLFCN_H) && defined(HAVE_DLADDR)
   // Use dladdr to get executable path if available.
   Dl_info DLInfo;
   int err = dladdr(MainAddr, &DLInfo);
   if (err == 0)
     return "";
 
   // If the filename is a symlink, we need to resolve and return the location of