Simplify the DenseSet used for hashing CodeView records.

This was storing the hash alongside the key so that the hash
doesn't need to be re-computed every time, but in doing so it
was allocating a structure to keep the key size small in the
DenseMap.  This is a noble goal, but it also leads to a pointer
indirection on every probe, and this cost of this pointer
indirection ends up being higher than the cost of having a
slightly larger entry in the hash table.  Removing this not only
simplifies the code, but yields a small but noticeable
performance improvement in the type merging algorithm.

llvm-svn: 319493