[libcxx] Define istream_iterator equality comparison operators out-of-line

Currently libc++ defines operator== and operator!= as friend functions in the
definition of the istream_iterator class template. Such definition has a subtle
difference from an out-of-line definition required by the C++ Standard: these
functions can only be found by argument-dependent lookup, but not by qualified
lookup.

This patch changes the definition, so that it conforms to the C++ Standard and
adds a check involving qualified lookup to the test suite.

Patch contributed by Mikhail Maltsev.

Differential Revision: https://reviews.llvm.org/D40415

llvm-svn: 320363