[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/SerializedModuleDependencyCacheFormat.h

@@ -37,7 +37,7 @@ 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 = 1;
+const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR = 2;
 /// Increment this on every change.
 const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MINOR = 0;
 
@@ -72,7 +72,8 @@ namespace graph_block {
 enum {
   METADATA = 1,
   MODULE_NODE,
-  SWIFT_TEXTUAL_MODULE_DETAILS_NODE,
+  SWIFT_INTERFACE_MODULE_DETAILS_NODE,
+  SWIFT_SOURCE_MODULE_DETAILS_NODE,
   SWIFT_PLACEHOLDER_MODULE_DETAILS_NODE,
   SWIFT_BINARY_MODULE_DETAILS_NODE,
   CLANG_MODULE_DETAILS_NODE,
@@ -109,7 +110,8 @@ using IdentifierArrayLayout =
 
 // After the array records, we have a sequence of Module info
 // records, each of which is followed by one of:
-// - SwiftTextualModuleDetails
+// - SwiftInterfaceModuleDetails
+// - SwiftSourceModuleDetails
 // - SwiftBinaryModuleDetails
 // - SwiftPlaceholderModuleDetails
 // - ClangModuleDetails
@@ -119,8 +121,8 @@ using ModuleInfoLayout =
                    DependencyIDArrayIDField // directDependencies
                    >;
 
-using SwiftTextualModuleDetailsLayout =
-    BCRecordLayout<SWIFT_TEXTUAL_MODULE_DETAILS_NODE, // ID
+using SwiftInterfaceModuleDetailsLayout =
+    BCRecordLayout<SWIFT_INTERFACE_MODULE_DETAILS_NODE, // ID
                    FileIDField,                       // swiftInterfaceFile
                    FileIDArrayIDField, // compiledModuleCandidates
                    FlagIDArrayIDField, // buildCommandLine
@@ -130,7 +132,16 @@ using SwiftTextualModuleDetailsLayout =
                    FileIDField,        // bridgingHeaderFile
                    FileIDArrayIDField, // sourceFiles
                    FileIDArrayIDField, // bridgingSourceFiles
-                   IdentifierIDField   // bridgingModuleDependencies
+                   FileIDArrayIDField   // bridgingModuleDependencies
+                   >;
+
+using SwiftSourceModuleDetailsLayout =
+    BCRecordLayout<SWIFT_SOURCE_MODULE_DETAILS_NODE, // ID
+                   FlagIDArrayIDField, // extraPCMArgs
+                   FileIDField,        // bridgingHeaderFile
+                   FileIDArrayIDField, // sourceFiles
+                   FileIDArrayIDField, // bridgingSourceFiles
+                   FileIDArrayIDField  // bridgingModuleDependencies
                    >;
 
 using SwiftBinaryModuleDetailsLayout =