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

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -124,7 +124,7 @@ void ExecutionEngine::addObjectFile(std::unique_ptr<object::ObjectFile> O) {
   llvm_unreachable("ExecutionEngine subclass doesn't implement addObjectFile.");
 }
 
-void ExecutionEngine::addArchive(std::unique_ptr<object::Archive> A) {
+void ExecutionEngine::addArchive(object::OwningBinary<object::Archive> A) {
   llvm_unreachable("ExecutionEngine subclass doesn't implement addArchive.");
 }