[libc++] Factor duplicate code into function templates

Summary:
The exact same code was replicated 11 times for implementing the basic_istream
input operators (those that don't use numeric_limits). The same code was also
duplicated twice for implementing the basic_istream input operators that take
numeric_limits into account.

This commit factors the common code into function templates to avoid
the duplication.

Reviewers: mclow.lists, EricWF

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D49808

llvm-svn: 337955