Declare some explicit instantiations for strings in Debug Mode

Declare some explicit instantiations for strings in Debug Mode

The empty reps and the I/O functions do not need to be implicitly
instantiated to enable assertions, so declare the explicit
instantiations when _GLIBCXX_EXTERN_TEMPLATE == -1 (i.e. when
_GLIBCXX_ASSERTIONS is defined).

Backport from mainline
2018-06-27  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/86138
	* include/bits/basic_string.tcc: [_GLIBCXX_EXTERN_TEMPLATE < 0]
	Declare explicit instantiations of COW empty reps and I/O functions.

From-SVN: r263383