Matthew Dempsky: Same as stdexcept.cpp in libc++abi: we've already computed 'len strlen(msg)', so we can use memcpy() instead of strcpy().

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185274 91177308-0d34-0410-b5e6-96231b3b80d8