Make the copy member of StringRef/ArrayRef generic wrt allocators.

Doesn't make sense to restrict this to BumpPtrAllocator. While there
replace an explicit loop with std::equal. Some standard libraries know
how to compile this down to a ::memcmp call if possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206615 91177308-0d34-0410-b5e6-96231b3b80d8