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

lib/Bitcode/Reader/BitcodeReader.h

@@ -221,7 +221,7 @@ public:
 
   void FreeState();
 
-  void releaseBuffer() override;
+  void releaseBuffer();
 
   bool isMaterializable(const GlobalValue *GV) const override;
   bool isDematerializable(const GlobalValue *GV) const override;