Refactor some of the operations in <string> so that they can be reused; no functionality change

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@196788 91177308-0d34-0410-b5e6-96231b3b80d8
Refactored a bunch of duplicated code in <ostream>. Made a new routine called __put_character_sequence, and made nine places call it.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@196951 91177308-0d34-0410-b5e6-96231b3b80d8