Have basic_istream::read call sgetn intead of sbumpc individual characters.  This addresses http://llvm.org/bugs/show_bug.cgi?id=15427.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@176573 91177308-0d34-0410-b5e6-96231b3b80d8