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

tools/verify-uselistorder/verify-uselistorder.cpp

@@ -157,7 +157,8 @@ std::unique_ptr<Module> TempFile::readBitcode(LLVMContext &Context) const {
   }
 
   MemoryBuffer *Buffer = BufferOr.get().get();
-  ErrorOr<Module *> ModuleOr = parseBitcodeFile(Buffer, Context);
+  ErrorOr<Module *> ModuleOr =
+      parseBitcodeFile(Buffer->getMemBufferRef(), Context);
   if (!ModuleOr) {
     DEBUG(dbgs() << "error: " << ModuleOr.getError().message() << "\n");
     return nullptr;