[flang] Fix standalone builds

[flang] Fix standalone builds

Flang's CMake modules directory was being added to the CMake module path
twice, and AddFlang was being included after the first addition. Remove
the unnecessary first addition and move the AddFlang include down to the
second one. This way, it occurs after LLVM's CMake modules have been
included for a standalone build, so it can make use of those modules.