Implement P0646R1: Erase-Like Algorithms Should Return size_type. Reviewed as...

Implement P0646R1: Erase-Like Algorithms Should Return size_type. Reviewed as https://reviews.llvm.org/D58332, and then updated because I rewrote a couple of those routines to eliminate some UB. Thanks to Zoe for tghe patch.

llvm-svn: 364840