libstdc++: Fix incorrect noexcept on string::compare overloads [PR123991]

libstdc++: Fix incorrect noexcept on string::compare overloads [PR123991]

These compare overloads throw when the pos index is out of range, not
only when the const T& parameter throws on conversion to string_view.

Remove the incorrect conditional noexcept-specifier from the two
overloads that can throw.

libstdc++-v3/ChangeLog:

	PR libstdc++/123991
	* include/bits/basic_string.h (compare(size_type, size_type, T)):
	Remove noexcept-specifier.
	(compare(size_type, size_type, T, size_type, size_type)):
	Likewise.
	* include/bits/cow_string.h (compare(size_type, size_type, T)):
	Remove noexcept-specifier.
	(compare(size_type, size_type, T, size_type, size_type)):
	Likewise.
	* testsuite/21_strings/basic_string/operations/compare/char/123991.cc:
	New test.
	* testsuite/21_strings/basic_string/operations/compare/wchar_t/123991.cc:
	New test.

Reviewed-by: Nathan Myers <nmyers@redhat.com>