Make ObjectFile and BitcodeReader always own the MemoryBuffer.

This allows us to just use a std::unique_ptr to store the pointer to the buffer.
The flip side is that they have to support releasing the buffer back to the
caller.

Overall this looks like a more efficient and less brittle api.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211542 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/GVMaterializer.h

@@ -54,6 +54,8 @@ public:
   /// Make sure the entire Module has been completely read.
   ///
   virtual std::error_code MaterializeModule(Module *M) = 0;
+
+  virtual void releaseBuffer() = 0;
 };
 
 } // End llvm namespace