[PATCH] [ELF] Use llvm ADT's instead of std.

[Sanjoy Das]

> These properties are not guaranteed by DenseMap. So, if you add a new item to a map while you are iterating over elements of the map, it may crash. It actually crashes only when the hash table is resized to add a new element for LHS, so it could produce a nasty flaky bug. For example the bug in the LayoutPass was there for more than 1 year until I fixed that.

A slight tangent:  do you guys think it will be worth the effort to
turn such errors to deterministically fail an assert in debug mode?
For instance, I can imagine having an "epoch" counter in the DenseMap
and DenseMapIterator, with modifications to the DenseMap bumping the
counter and iterator accesses asserting "ParentDenseMap->epoch ==
this->epoch" on access.

>
> Even
>
> m[x] = m[y]
>
> is not guaranteed to be safe for a DenseMap m, because a reference returned by m[y] can be invalidated by m[x]. This particular one is the bug in LayoutPass.cpp that Reid mentioned above (fix is r213969).
>
> I checked the code quickly, and it looks safe to me, but please review your patch again with the above thing in your mind.
>
>
> http://reviews.llvm.org/D7885
>
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