string_view: Rep empty string with unit-length static constexpr string.

string_view: Rep empty string with unit-length static constexpr string.

2013-11-21  Edward Smith-Rowland  <3dw4rd@verizon.net>

	* include/experimental/string_view: Rep empty string with unit-length
	static constexpr string. Uncomment _GLIBCXX_VISIBILITY. Enforce
	invariant of no nullptr string pointer.
	* include/experimental/string_view.tcc: Ditto.
	* testsuite/experimental/string_view/cons/char/1.cc: data() for empty
	string_view is no longer nullptr.
	* testsuite/experimental/string_view/cons/wchar_t/1.cc: Ditto.
	* testsuite/experimental/string_view/operations/data/char/1.cc: Ditto.
	* testsuite/experimental/string_view/operations/data/wchar_t/1.cc:
	Ditto.

From-SVN: r205213