[libc++] Fix hardening checks in std::string operator[]

[winner245]

The current valid-element-access hardening checks in the operator[] for basic_string are improperly implemented, which do not trap accesses one past the end of the string. This PR corrects the issue and adds corresponding assertion tests to validate the checks.

[frederick-vs-ja]

No... I believe the existing checks are correct, because the standard specifies that s[s.size()] is well-defined and can even refer to a writable object! Although a program is only permitted to write CharT() to it. (Unfortunately, I have no idea how to check this.)

See

• https://en.cppreference.com/w/cpp/string/basic_string/data
• [string.access]/2
• [string.accessors]