[RuntimeDyld] DenseMap -> std::unordered_map

DenseMap is most applicable when both keys and values are small.
In this case, the value violates that assumption, causing quite
significant memory overhead. A std::unordered_map is more appropriate
in this case (or at least fixed the memory problems I was seeing).

Differential Revision: http://reviews.llvm.org/D14910

llvm-svn: 254651