Improve key-function computation for templates. In particular:

- All classes can have a key function; templates don't change that.
  non-template classes when computing the key function.
  - We always mark all of the virtual member functions of class
  template instantiations. 
  - The vtable for an instantiation of a class template has weak
  linkage. 

We could probably use available_externally linkage for vtables of
classes instantiated by explicit instantiation declarations (extern
templates), but GCC doesn't do this and I'm not 100% that the ABI
permits it.

llvm-svn: 92753

clang/lib/CodeGen/CodeGenModule.cpp

@@ -547,7 +547,8 @@ bool CodeGenModule::MayDeferGeneration(const ValueDecl *Global) {
       const CXXRecordDecl *RD = MD->getParent();
       if (MD->isOutOfLine() && RD->isDynamicClass()) {
         const CXXMethodDecl *KeyFunction = getContext().getKeyFunction(RD);
-        if (KeyFunction == MD->getCanonicalDecl())
+        if (KeyFunction && 
+            KeyFunction->getCanonicalDecl() == MD->getCanonicalDecl())
           return false;
       }
     }