[ParseableInterface] Only open module buffers once while loading

In addition to being wasteful, this is a correctness issue -- the
compiler should only ever have one view of this file, and it should not
read a potentially different file after validating dependencies.

rdar://48654608

include/swift/Serialization/SerializedModuleLoader.h

@@ -63,6 +63,11 @@ protected:
                   std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
                   std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer);
 
+  std::error_code
+  openModuleDocFile(AccessPathElem ModuleID,
+                    StringRef ModuleDocPath,
+                    std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer);
+
   /// If the module loader subclass knows that all options have been tried for
   /// loading an architecture-specific file out of a swiftmodule bundle, try
   /// to list the architectures that \e are present.