[Serialization] Preserve @_implementationOnly through module merging

When we build incrementally, we produce "partial swiftmodules" for
each input source file, then merge them together into the final
compiled module that, among other things, gets used for debugging.
Without this, we'd drop @_implementationOnly imports and any types
from the modules that were imported during the module-merging step
and then be unable to debug those types

include/swift/Serialization/ModuleFile.h

@@ -659,11 +659,26 @@ public:
   // Out of line to avoid instantiation OnDiskChainedHashTable here.
   ~ModuleFile();
 
-  /// Associates this module file with an AST module.
+  /// Associates this module file with the AST node representing it.
   ///
-  /// Returns any error that occurred during association, including validation
-  /// that the module file is compatible with the module it's being loaded as.
-  Status associateWithFileContext(FileUnit *file, SourceLoc diagLoc);
+  /// Checks that the file is compatible with the AST module it's being loaded
+  /// into, loads any dependencies needed to understand the module, and updates
+  /// the ASTContext and ClangImporter with search paths and other information
+  /// from the module.
+  ///
+  /// \param file The FileUnit that represents this file's place in the AST.
+  /// \param diagLoc A location used for diagnostics that occur during loading.
+  /// This does not include diagnostics about \e this file failing to load,
+  /// but rather other things that might be imported as part of bringing the
+  /// file into the AST.
+  /// \param treatAsPartialModule If true, processes implementation-only
+  /// information instead of assuming the client won't need it and shouldn't
+  /// see it.
+  ///
+  /// \returns any error that occurred during association, such as being
+  /// compiled for a different OS.
+  Status associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
+                                  bool treatAsPartialModule);
 
   /// Checks whether this module can be used.
   Status getStatus() const {