First part of P1227R2 - change span over to use 'size_t' instead of 'ptrdiff_t'. Reviewed as https://reviews.llvm.org/D58639.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@354936 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed