[Dependency Scanning] Attempt to lookup optional transitive dependencies of binary module dependencies. Instead of simply pretending they do not exist, do a best-effort lookup

include/swift/DependencyScan/SerializedModuleDependencyCacheFormat.h

@@ -39,7 +39,7 @@ using llvm::BCVBR;
 
 /// Every .moddepcache file begins with these 4 bytes, for easy identification.
 const unsigned char MODULE_DEPENDENCY_CACHE_FORMAT_SIGNATURE[] = {'I', 'M', 'D','C'};
-const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR = 4;
+const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR = 5; // optionalModuleImports
 /// Increment this on every change.
 const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MINOR = 1;
 
@@ -124,6 +124,7 @@ using ModuleInfoLayout =
                    IdentifierIDField,            // moduleName
                    ContextHashIDField,           // contextHash
                    ImportArrayIDField,           // moduleImports
+                   ImportArrayIDField,           // optionalModuleImports
                    DependencyIDArrayIDField      // resolvedDirectModuleDependencies
                    >;