Fixing a FIXME -- no longer using std::memcpy, since that would fail for non-trivial types. Replaced with std::copy. No functional changes intended since all uses of this functionality either use pointers or integers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207766 91177308-0d34-0410-b5e6-96231b3b80d8