Recommit 213307: unique_ptr-ify ownership of ASTConsumers (reverted in r213325)

After post-commit review and community discussion, this seems like a
reasonable direction to continue, making ownership semantics explicit in
the source using the type system.

llvm-svn: 215323
diff --git a/clang/lib/Frontend/ASTMerge.cpp b/clang/lib/Frontend/ASTMerge.cpp
index 95cfe248..216ac6a 100644
--- a/clang/lib/Frontend/ASTMerge.cpp
+++ b/clang/lib/Frontend/ASTMerge.cpp
@@ -16,8 +16,8 @@
 
 using namespace clang;
 
-ASTConsumer *ASTMergeAction::CreateASTConsumer(CompilerInstance &CI,
-                                               StringRef InFile) {
+std::unique_ptr<ASTConsumer>
+ASTMergeAction::CreateASTConsumer(CompilerInstance &CI, StringRef InFile) {
   return AdaptedAction->CreateASTConsumer(CI, InFile);
 }