[clang][deps] Sort submodules when calculating dependencies

Dependency scanning does not care about the order of submodules for
correctness, so sort the submodules so that we get the same
command-lines to build the module across different TUs. The order of
inferred submodules can vary depending on the order of #includes in the
including TU.

Differential Revision: https://reviews.llvm.org/D128008