[clang][Modules] `checkModuleIsAvailable` should use a const & parameter...

[clang][Modules] `checkModuleIsAvailable` should use a const & parameter instead of pointer (#67902)

The `Module` parameter to `checkModuleIsAvailable` is currently passed
by pointer to non-const. However, it requires only const access and it
cannot be null. Change this to be a reference to const instead.

This then makes it obvious that it is an input-only parameter, so move
it to be before the in-out parameter for diagnostics.