Don't own the buffer in object::Binary.

Owning the buffer is somewhat inflexible. Some Binaries have sub Binaries
(like Archive) and we had to create dummy buffers just to handle that. It is
also a bad fit for IRObjectFile where the Module wants to own the buffer too.

Keeping this ownership would make supporting IR inside native objects
particularly painful.

This patch focuses in lib/Object. If something elsewhere used to own an Binary,
now it also owns a MemoryBuffer.

This patch introduces a few new types.

* MemoryBufferRef. This is just a pair of StringRefs for the data and name.
  This is to MemoryBuffer as StringRef is to std::string.
* OwningBinary. A combination of Binary and a MemoryBuffer. This is needed
  for convenience functions that take a filename and return both the
  buffer and the Binary using that buffer.

The C api now uses OwningBinary to avoid any change in semantics. I will start
a new thread to see if we want to change it and how.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216002 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ExecutionEngine/ObjectCache.h

@@ -27,7 +27,7 @@ public:
   virtual ~ObjectCache() { }
 
   /// notifyObjectCompiled - Provides a pointer to compiled code for Module M.
-  virtual void notifyObjectCompiled(const Module *M, const MemoryBuffer *Obj) = 0;
+  virtual void notifyObjectCompiled(const Module *M, MemoryBufferRef Obj) = 0;
 
   /// Returns a pointer to a newly allocated MemoryBuffer that contains the
   /// object which corresponds with Module M, or 0 if an object is not