[PATCH] PR libstdc++/83511 add default argument to basic_string_view::substr

[Jonathan Wakely]

PR libstdc++/83511
* include/std/string_view (basic_string_view::substr): Add default
argument to first parameter.
* include/experimental/string_view (basic_string_view::substr):
Likewise.
* testsuite/21_strings/basic_string_view/operations/substr/char/
83511.cc: New test.
* testsuite/21_strings/basic_string_view/operations/substr/wchar_t/
83511.cc: New test.
* testsuite/experimental/string_view/operations/substr/char/83511.cc:
New test.
* testsuite/experimental/string_view/operations/substr/wchar_t/83511.cc:
New test.

Tested powerpc64le-linux, committed to trunk.