[modularize] Stabilize iteration order when processing module maps

Many diagnostics (e.g., ProblemsDuplicate.modularize,
ProblemsDisplayLists.modularize) are dependent on the iteration order of
StringMap, which is not guaranteed to be deterministic
(https://llvm.org/docs/ProgrammersManual.html#llvm-adt-stringmap-h).
clang::ModuleMap::Modules is a StringMap. For now, sort by name in
modularize.