libstdc++: Make std::istream_iterator copy ctor constexpr (LWG 3600)

As explained in LWG 3600, we never implemented a C++0x change that made
the copy constructor of std::istream_iterator defined as defaulted. That
would be an ABI break, so the resolution of LWG 3600 is to not require
it to be trivial, but just constexpr and conditionally noexcept. This
applies that resolution.

libstdc++-v3/ChangeLog:

	* include/bits/stream_iterator.h (istream_iterator): Add
	constexpr to copy constructor, as per LWG 3600.
	* testsuite/24_iterators/istream_iterator/cons/constexpr.cc:
	Check copy construction.