[ORC] Modify DenseMap hashing for SymbolStringPtrs.

[ORC] Modify DenseMap hashing for SymbolStringPtrs.

Modifies the DenseMapInfo<SymbolStringPtr>::getHashValue method to take its
argument by const-ref rather than by value (to avoid unnecessary ref-counting
operations) and to defer to DenseMapInfo<void*> for the hash value computation
(since SymbolStringPtrs are just pointers under the hood).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360831 91177308-0d34-0410-b5e6-96231b3b80d8