[modules] Correctly overload getModule in the MultiplexExternalSemaSource

[teemperor]

Summary

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

[teemperor]

• Removed virtual keyword when we already have override.

[v.g.vassilev]

LGTM!

include/clang/Sema/MultiplexExternalSemaSource.h

public:
 
   /// \brief Function that will be invoked when we begin parsing a new
   /// translation unit involving this external AST source.
   void StartTranslationUnit(ASTConsumer *Consumer) override;
 
   /// \brief Print any statistics that have been gathered regarding
   /// 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.
   ///
   /// This routine allows the external AST source to provide an specific 
   /// layout for a record, overriding the layout that would normally be
   /// constructed. It is intended for clients who receive specific layout
   /// details rather than source code (such as LLDB). The client is expected
   /// to fill in the field offsets, base offsets, virtual base offsets, and
   /// complete object size.

lib/Sema/MultiplexExternalSemaSource.cpp

for(size_t i = 0; i < Sources.size(); ++i)
     Sources[i]->StartTranslationUnit(Consumer);
 }
 
 void MultiplexExternalSemaSource::PrintStats() {
   for(size_t i = 0; i < Sources.size(); ++i)
     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,
                       llvm::DenseMap<const FieldDecl *, uint64_t> &FieldOffsets,
                   llvm::DenseMap<const CXXRecordDecl *, CharUnits> &BaseOffsets,
           llvm::DenseMap<const CXXRecordDecl *, CharUnits> &VirtualBaseOffsets){
   for(size_t i = 0; i < Sources.size(); ++i)
     if (Sources[i]->layoutRecordType(Record, Size, Alignment, FieldOffsets,