[Dependency Scanning] Scan header inputs of binary Swift moduel dependencies

Otherwise they may have module dependencies of their own which will not be detected by the scanner and included in the list of explicit inputs for compilation.

include/swift/Serialization/SerializedModuleLoader.h

@@ -168,7 +168,7 @@ protected:
 
   struct BinaryModuleImports {
     llvm::StringSet<> moduleImports;
-    llvm::StringSet<> headerImports;
+    std::string headerImport;
   };
 
   static llvm::ErrorOr<BinaryModuleImports>