First part of P1024: Usability Enhancements for std::span. Remove operator() for indexing, and add 'front' and 'back' calls.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@354801 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed