[ASTReader] Add a DeserializationListener callback for IMPORTED_MODULES

[graydon]

Summary

Add a callback from ASTReader to DeserializationListener when the former
reads an IMPORTED_MODULES block. This supports Swift in using PCH for
bridging headers.

[manmanren]

LGTM

Manman

include/clang/Serialization/ASTDeserializationListener.h

 namespace clang {
 
 class Decl;
 class ASTReader;
 class QualType;
 class MacroDefinitionRecord;
 class MacroInfo;
 class Module;
+class SourceLocation;
 
 class ASTDeserializationListener {
 public:
   virtual ~ASTDeserializationListener();
 
   /// \brief The ASTReader was initialized.
   virtual void ReaderInitialized(ASTReader *Reader) { }
 
public:
   virtual void DeclRead(serialization::DeclID ID, const Decl *D) { }
   /// \brief A selector was read from the AST file.
   virtual void SelectorRead(serialization::SelectorID iD, Selector Sel) {}
   /// \brief A macro definition was read from the AST file.
   virtual void MacroDefinitionRead(serialization::PreprocessedEntityID,
                                    MacroDefinitionRecord *MD) {}
   /// \brief A module definition was read from the AST file.
   virtual void ModuleRead(serialization::SubmoduleID ID, Module *Mod) {}
+  /// \brief A module import was read from the AST file.
+  virtual void ModuleImportRead(serialization::SubmoduleID ID,
+                                SourceLocation ImportLoc) {}
 };
 }
 
 #endif

lib/Serialization/ASTReader.cpp

while (true) {
     case IMPORTED_MODULES: {
       if (!F.isModule()) {
         // If we aren't loading a module (which has its own exports), make
         // all of the imported modules visible.
         // FIXME: Deal with macros-only imports.
         for (unsigned I = 0, N = Record.size(); I != N; /**/) {
           unsigned GlobalID = getGlobalSubmoduleID(F, Record[I++]);
           SourceLocation Loc = ReadSourceLocation(F, Record, I);
-          if (GlobalID)
+          if (GlobalID) {
             ImportedModules.push_back(ImportedSubmodule(GlobalID, Loc));
+            if (DeserializationListener)
+              DeserializationListener->ModuleImportRead(GlobalID, Loc);
+          }
         }
       }
       break;
     }
 
     case MACRO_OFFSET: {
       if (F.LocalNumMacros != 0) {
         Error("duplicate MACRO_OFFSET record in AST file");