Implement P0403R1, Literal suffixes for basic_string_view.

Implement P0403R1, Literal suffixes for basic_string_view.

* include/std/string_view
(operator""sv(const char*, size_t)): New.
(operator""sv(const wchar_t*, size_t)): Likewise.
(operator""sv(const char16_t*, size_t)): Likewise.
(operator""sv(const char32_t*, size_t)): Likewise.
* testsuite/21_strings/basic_string_view/literals/types.cc: New.
* testsuite/21_strings/basic_string_view/literals/values.cc: Likewise.
* testsuite/experimental/string_view/literals/values.cc: Add
tests for literals with embedded NULs.

From-SVN: r242367