Adapt and integrate string_view tests

Adapt and integrate string_view tests

The previous patch copied the string_view tests from libstdc++.  This
patch adjusts them in a similar way that the libstdc++ optional tests
are integrated in our unit test suite.

Not all tests are used, some of them require language features not
present in c++11.  For example, we can't use a string_view constructor
where the length is not explicit in a constexpr, because
std::char_traits::length is not a constexpr itself (it is in c++17
though).  Nevertheless, a good number of tests are integrated, which
covers pretty well the string_view features.

gdb/ChangeLog:

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	string_view-selftests.c.
	* unittests/basic_string_view/capacity/1.cc: Adapt to GDB
	testsuite.
	* unittests/basic_string_view/cons/char/1.cc: Likewise.
	* unittests/basic_string_view/cons/char/2.cc: Likewise.
	* unittests/basic_string_view/cons/char/3.cc: Likewise.
	* unittests/basic_string_view/element_access/char/1.cc:
	Likewise.
	* unittests/basic_string_view/element_access/char/empty.cc:
	Likewise.
	* unittests/basic_string_view/element_access/char/front_back.cc:
	Likewise.
	* unittests/basic_string_view/inserters/char/2.cc: Likewise.
	* unittests/basic_string_view/modifiers/remove_prefix/char/1.cc:
	Likewise.
	* unittests/basic_string_view/modifiers/remove_suffix/char/1.cc:
	Likewise.
	* unittests/basic_string_view/modifiers/swap/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operations/compare/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operations/compare/char/13650.cc:
	Likewise.
	* unittests/basic_string_view/operations/copy/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operations/data/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operations/find/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operations/find/char/2.cc:
	Likewise.
	* unittests/basic_string_view/operations/find/char/3.cc:
	Likewise.
	* unittests/basic_string_view/operations/find/char/4.cc:
	Likewise.
	* unittests/basic_string_view/operations/rfind/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operations/rfind/char/2.cc:
	Likewise.
	* unittests/basic_string_view/operations/rfind/char/3.cc:
	Likewise.
	* unittests/basic_string_view/operations/substr/char/1.cc:
	Likewise.
	* unittests/basic_string_view/operators/char/2.cc: Likewise.
	* unittests/string_view-selftests.c: New file.