[ADT] Bring back memmove to make GCC 5.4 happy

All other GCCs look good so far. GCC 5.4 complains about strict
aliasing, so fix that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342643 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed