[Dependency Scanning] Model main module as separate dependency kind: SwiftSource

These kinds of modules differ from `SwiftTextual` modules in that they do not have an interface and have source-files.
It is cleaner to enforce this distinction with types, instead of checking for interface optionality everywhere.

include/swift/DependencyScan/StringUtils.h

@@ -35,6 +35,9 @@ swiftscan_string_set_t *create_set(const std::vector<std::string> &strings);
 /// create_clone routine.
 swiftscan_string_set_t *create_set(int count, const char **strings);
 
+/// Create an empty array of swiftscan_string_ref_t objects
+swiftscan_string_set_t *create_empty_set();
+
 /// Retrieve the character data associated with the given string.
 const char *get_C_string(swiftscan_string_ref_t string);
 }