Optimize MDNode to coallocate the operand list immediately

Optimize MDNode to coallocate the operand list immediately
after the MDNode in memory.  This eliminates the operands
pointer and saves a new[] per node.

Note that the code in DIDerivedType::replaceAllUsesWith is wrong
and quite scary.  A MDNode should not be RAUW'd with something
else: this changes all uses of the mdnode, which may not be debug
info related!  Debug info should use something non-mdnode for
declarations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92321 91177308-0d34-0410-b5e6-96231b3b80d8