Numeric parsing was getting the wrong answer when faced with very long inputs.  This fixes both http://llvm.org/bugs/show_bug.cgi?id=15751 and http://llvm.org/bugs/show_bug.cgi?id=15740

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@179556 91177308-0d34-0410-b5e6-96231b3b80d8