[Dependency Scanning] Keep track of each imported module's access control

Adds an access control field for each imported module identified. When multiple imports of the same module are found, this keeps track of the most "open" access specifier.

include/swift/Serialization/SerializedModuleLoader.h

@@ -170,8 +170,7 @@ protected:
                  bool isTestableImport, bool isCandidateForTextualModule);
 
   struct BinaryModuleImports {
-    llvm::StringSet<> moduleImports;
-    llvm::StringSet<> exportedModules;
+    std::vector<ScannerImportStatementInfo> moduleImports;
     std::string headerImport;
   };