[Dependency Scanning] Introduce a new constructor for dummy source modules

Instead of passing in a bunch of dummy/empty data.

Author: artemcm

include/swift-c/DependencyScan/DependencyScan.h

@@ -475,6 +475,17 @@ swiftscan_source_location_get_line_number(swiftscan_source_location_t source_loc
 SWIFTSCAN_PUBLIC int64_t
 swiftscan_source_location_get_column_number(swiftscan_source_location_t source_location);
 
+//=== Scanner Cache Operations --------------------------------------------===//
+// The following operations expose an implementation detail of the dependency
+// scanner: its module dependencies cache. This is done in order
+// to allow clients to perform incremental dependency scans by having the
+// scanner's state be serializable and re-usable.
+
+/// For the specified \c scanner instance, reset its internal state, ensuring subsequent
+/// scanning queries are done "from-scratch".
+SWIFTSCAN_PUBLIC void
+swiftscan_scanner_cache_reset(swiftscan_scanner_t scanner);
+
 /// An entry point to invoke the compiler via a library call.
 SWIFTSCAN_PUBLIC int invoke_swift_compiler(int argc, const char **argv);
 

include/swift/AST/ModuleDependencies.h

@@ -633,6 +633,17 @@ class ModuleDependencyInfo {
             extraPCMArgs));
   }
 
+  static ModuleDependencyInfo
+  forSwiftSourceModule() {
+    return ModuleDependencyInfo(
+        std::make_unique<SwiftSourceModuleDependenciesStorage>(
+             StringRef(), ArrayRef<StringRef>(),
+             ArrayRef<ScannerImportStatementInfo>(),
+             ArrayRef<ScannerImportStatementInfo>(),
+             ArrayRef<StringRef>(),
+             ArrayRef<StringRef>()));
+  }
+
   /// Describe the module dependencies for a Clang module that can be
   /// built from a module map and headers.
   static ModuleDependencyInfo forClangModule(

lib/AST/ModuleDependencies.cpp

@@ -784,7 +784,7 @@ ModuleDependenciesCache::findDependency(
     auto dep = optionalDep.value();
     if (dep->getAsSwiftSourceModule() &&
         moduleName != mainScanModuleName &&
-        moduleName != "DummyMainModuleForResolvingCrossImportOverlays") {
+        moduleName != "MainModuleCrossImportOverlays") {
       return std::nullopt;
     }
   }

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -1161,13 +1161,13 @@ void ModuleDependencyScanner::discoverCrossImportOverlayDependencies(
 
   // Construct a dummy main to resolve the newly discovered cross import
   // overlays.
-  StringRef dummyMainName = "DummyMainModuleForResolvingCrossImportOverlays";
+  StringRef dummyMainName = "MainModuleCrossImportOverlays";
   auto dummyMainID = ModuleDependencyID{dummyMainName.str(),
                                         ModuleDependencyKind::SwiftSource};
   auto actualMainID = ModuleDependencyID{mainModuleName.str(),
                                          ModuleDependencyKind::SwiftSource};
   auto dummyMainDependencies =
-     ModuleDependencyInfo::forSwiftSourceModule({}, {}, {}, {}, {}, {});
+     ModuleDependencyInfo::forSwiftSourceModule();
   std::for_each(newOverlays.begin(), newOverlays.end(),
                 [&](Identifier modName) {
                   dummyMainDependencies.addModuleImport(modName.str());