Make rehash(0) work with ubsan's unsigned-integer-overflow.
Make rehash(0) work with ubsan's unsigned-integer-overflow.

Reviewers: mclow.lists, EricWF

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40743

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@322031 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx