AsmPrinter: Document why DIEValueList uses a linked-list, NFC

There are two main reasons why a linked-list makes sense for
`DIEValueList`.

 1. We want `DIE` to be on a `BumpPtrAllocator` to improve teardown
    efficiency.  Making `DIEValueList` array-based would make that much
    more complicated.
 2. The singly-linked list is fairly memory efficient.  The histogram
    [1] shows that most DIEs have relatively few values, so we often pay
    less than the 2/3-pointer static overhead of a vector.  Furthermore,
    we don't know ahead of time exactly how many values a `DIE` needs,
    so a vector-like scheme will on average over-allocate by ~50%.  As
    it happens, that's the same memory overhead as the linked list node.

[1]: http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-May/085910.html

The comment I added to the code is a little more succinct, but I think
it's enough to give the idea.

llvm-svn: 240868

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