[DenseMapInfo] Move hash_code implementation to Hashing.h (NFC)

This moves the DenseMapInfo implementation for hash_code into
Hashing.h, removing the need to include Hashing.h (and thus <string>)
in DenseMapInfo.h. This follows the general convention of declaring
DenseMapInfo for types that we own in the respective header. The
remaining implementations in DenseMapInfo.h are all for types we
do not own.

Differential Revision: https://reviews.llvm.org/D111451