[windows] Use a well-known path for ComSpec if we fail to retrieve it

[stella.stamenova]

Summary

Right now we always try to retrieve ComSpec and if we fail, we give up. This rarely fails, but we can update the logic so that we fail even less frequently. Since there is a well-known path (albeit not always correct), try the path when we failed to retrieve it. Note that on other platforms, we generally just return a well-known path without any checking.

source/Host/windows/HostInfoWindows.cpp

llvm::call_once(g_once_flag, []() {
     std::string path;
     llvm::convertWideToUTF8(buffer.data(), path);
     m_program_filespec.SetFile(path, false, FileSpec::Style::native);
   });
   return m_program_filespec;
 }
 
 FileSpec HostInfoWindows::GetDefaultShell() {
+  // Try to retrieve ComSpec from the environment. On the rare occasion
+  // that it fails, try a well-known path for ComSpec instead.
+
   std::string shell;
-  GetEnvironmentVar("ComSpec", shell);
+  if (GetEnvironmentVar("ComSpec", shell))
     return FileSpec(shell, false);
+
+  return FileSpec("C:\\Windows\\system32\\cmd.exe", false);
 }
 
 bool HostInfoWindows::GetEnvironmentVar(const std::string &var_name,
                                         std::string &var) {
   std::wstring wvar_name;
   if (!llvm::ConvertUTF8toWide(var_name, wvar_name))
     return false;
 
   if (const wchar_t *wvar = _wgetenv(wvar_name.c_str()))
     return llvm::convertWideToUTF8(wvar, var);
   return false;
 }