Return a std::unique_ptr from getBufferForFile. NFC.

llvm-svn: 216476
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 8101732..b5f1b4f 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -638,7 +638,7 @@
 llvm::MemoryBuffer *ASTUnit::getBufferForFile(StringRef Filename,
                                               std::string *ErrorStr) {
   assert(FileMgr);
-  return FileMgr->getBufferForFile(Filename, ErrorStr);
+  return FileMgr->getBufferForFile(Filename, ErrorStr).release();
 }
 
 /// \brief Configure the diagnostics object for use with ASTUnit.