[clang] Make access to submodules via `iterator_range`

In file `clang/lib/Basic/Module.cpp` the `Module` class had `submodule_begin()` and `submodule_end()` functions to retrieve corresponding iterators for private vector of Modules. This commit removes mentioned functions, and replaces all of theirs usages with `submodules()` function and range-based for-loops.

Differential Revision: https://reviews.llvm.org/D148954