Copy string_view tests from libstdc++

Copy string_view tests from libstdc++

This patch copies the string_view tests from the gcc repository (commit
02a4441f002c).

  ${gcc}/libstdc++-v3/testsuite/21_strings/basic_string_view ->
    ${binutils-gdb}/gdb/unittests/basic_string_view

The local modifications are done in the following patch, so that it's
easier to review them.

gdb/ChangeLog:

	* unittests/basic_string_view/capacity/1.cc: New file.
	* unittests/basic_string_view/capacity/empty_neg.cc: New file.
	* unittests/basic_string_view/cons/char/1.cc: New file.
	* unittests/basic_string_view/cons/char/2.cc: New file.
	* unittests/basic_string_view/cons/char/3.cc: New file.
	* unittests/basic_string_view/cons/wchar_t/1.cc: New file.
	* unittests/basic_string_view/cons/wchar_t/2.cc: New file.
	* unittests/basic_string_view/cons/wchar_t/3.cc: New file.
	* unittests/basic_string_view/element_access/char/1.cc: New file.
	* unittests/basic_string_view/element_access/char/2.cc: New file.
	* unittests/basic_string_view/element_access/char/empty.cc: New file.
	* unittests/basic_string_view/element_access/char/front_back.cc: New file.
	* unittests/basic_string_view/element_access/wchar_t/1.cc: New file.
	* unittests/basic_string_view/element_access/wchar_t/2.cc: New file.
	* unittests/basic_string_view/element_access/wchar_t/empty.cc: New file.
	* unittests/basic_string_view/element_access/wchar_t/front_back.cc: New file.
	* unittests/basic_string_view/include.cc: New file.
	* unittests/basic_string_view/inserters/char/1.cc: New file.
	* unittests/basic_string_view/inserters/char/2.cc: New file.
	* unittests/basic_string_view/inserters/char/3.cc: New file.
	* unittests/basic_string_view/inserters/pod/10081-out.cc: New file.
	* unittests/basic_string_view/inserters/wchar_t/1.cc: New file.
	* unittests/basic_string_view/inserters/wchar_t/2.cc: New file.
	* unittests/basic_string_view/inserters/wchar_t/3.cc: New file.
	* unittests/basic_string_view/literals/types.cc: New file.
	* unittests/basic_string_view/literals/values.cc: New file.
	* unittests/basic_string_view/modifiers/remove_prefix/char/1.cc: New file.
	* unittests/basic_string_view/modifiers/remove_prefix/wchar_t/1.cc: New file.
	* unittests/basic_string_view/modifiers/remove_suffix/char/1.cc: New file.
	* unittests/basic_string_view/modifiers/remove_suffix/wchar_t/1.cc: New file.
	* unittests/basic_string_view/modifiers/swap/char/1.cc: New file.
	* unittests/basic_string_view/modifiers/swap/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operations/compare/char/1.cc: New file.
	* unittests/basic_string_view/operations/compare/char/13650.cc: New file.
	* unittests/basic_string_view/operations/compare/char/2.cc: New file.
	* unittests/basic_string_view/operations/compare/char/70483.cc: New file.
	* unittests/basic_string_view/operations/compare/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operations/compare/wchar_t/13650.cc: New file.
	* unittests/basic_string_view/operations/compare/wchar_t/2.cc: New file.
	* unittests/basic_string_view/operations/copy/char/1.cc: New file.
	* unittests/basic_string_view/operations/copy/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operations/data/char/1.cc: New file.
	* unittests/basic_string_view/operations/data/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operations/find/char/1.cc: New file.
	* unittests/basic_string_view/operations/find/char/2.cc: New file.
	* unittests/basic_string_view/operations/find/char/3.cc: New file.
	* unittests/basic_string_view/operations/find/char/4.cc: New file.
	* unittests/basic_string_view/operations/find/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operations/find/wchar_t/2.cc: New file.
	* unittests/basic_string_view/operations/find/wchar_t/3.cc: New file.
	* unittests/basic_string_view/operations/find/wchar_t/4.cc: New file.
	* unittests/basic_string_view/operations/rfind/char/1.cc: New file.
	* unittests/basic_string_view/operations/rfind/char/2.cc: New file.
	* unittests/basic_string_view/operations/rfind/char/3.cc: New file.
	* unittests/basic_string_view/operations/rfind/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operations/rfind/wchar_t/2.cc: New file.
	* unittests/basic_string_view/operations/rfind/wchar_t/3.cc: New file.
	* unittests/basic_string_view/operations/string_conversion/1.cc: New file.
	* unittests/basic_string_view/operations/substr/char/1.cc: New file.
	* unittests/basic_string_view/operations/substr/wchar_t/1.cc: New file.
	* unittests/basic_string_view/operators/char/2.cc: New file.
	* unittests/basic_string_view/operators/wchar_t/2.cc: New file.
	* unittests/basic_string_view/range_access/char/1.cc: New file.
	* unittests/basic_string_view/range_access/wchar_t/1.cc: New file.
	* unittests/basic_string_view/requirements/explicit_instantiation/1.cc: New file.
	* unittests/basic_string_view/requirements/explicit_instantiation/char/1.cc: New file.
	* unittests/basic_string_view/requirements/explicit_instantiation/char16_t/1.cc: New file.
	* unittests/basic_string_view/requirements/explicit_instantiation/char32_t/1.cc: New file.
	* unittests/basic_string_view/requirements/explicit_instantiation/wchar_t/1.cc: New file.
	* unittests/basic_string_view/requirements/typedefs.cc: New file.
	* unittests/basic_string_view/typedefs.cc: New file.
	* unittests/basic_string_view/types/1.cc: New file.