[modules] Correctly overload getModule in the MultiplexExternalSemaSource

Summary:
The MultiplexExternalSemaSource doesn't correctly overload the `getModule` function,
causing the multiplexer to not forward this call as intended.

Reviewers: v.g.vassilev

Reviewed By: v.g.vassilev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39416

llvm-svn: 323122

clang/include/clang/Sema/MultiplexExternalSemaSource.h

@@ -149,6 +149,8 @@ public:
   /// the external AST source.
   void PrintStats() override;
 
+  /// \brief Retrieve the module that corresponds to the given module ID.
+  Module *getModule(unsigned ID) override;
 
   /// \brief Perform layout on the given record.
   ///

clang/lib/Sema/MultiplexExternalSemaSource.cpp

@@ -164,6 +164,13 @@ void MultiplexExternalSemaSource::PrintStats() {
     Sources[i]->PrintStats();
 }
 
+Module *MultiplexExternalSemaSource::getModule(unsigned ID) {
+  for (size_t i = 0; i < Sources.size(); ++i)
+    if (auto M = Sources[i]->getModule(ID))
+      return M;
+  return nullptr;
+}
+
 bool MultiplexExternalSemaSource::layoutRecordType(const RecordDecl *Record,
                                                    uint64_t &Size, 
                                                    uint64_t &Alignment,