Pass a MemoryBufferRef when we can avoid taking ownership.

The attached patch simplifies a few interfaces that don't need to take
ownership of a buffer.

For example, both parseAssembly and parseBitcodeFile will parse the
entire buffer before returning. There is no need to take ownership.

Using a MemoryBufferRef makes it obvious in the type signature that
there is no ownership transfer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216488 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/GVMaterializer.h

@@ -54,8 +54,6 @@ 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