[ADT] Add a 'find_as' operation to DenseSet.

This operation is analogous to its counterpart in DenseMap: It allows lookup
via cheap-to-construct keys (provided that getHashValue and isEqual are
implemented for the cheap key-type in the DenseMapInfo specialization).

Thanks to Chandler for the review.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220168 91177308-0d34-0410-b5e6-96231b3b80d8