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

Seems to have broken some tests since I first wrote this a while
back. Will reland after checking what went wrong with the tests.

This reverts commit 7023194c8d11a081fd01ed25308b3d60193c6a06.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322039 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed