Make the helper routines in string really be constexpr. This required a bit of refacoring in algorithm as well. Give them better names while we're at it. All of these are internal rotines; no visible functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210561 91177308-0d34-0410-b5e6-96231b3b80d8