[libc++] Fixes basic_string operator& hijacking.

Avoids using operator& in basic_string since an evil char-like type can
hijack this operator. Added some more evil operators, this found a place
where equality was compared directly and not via the traits.

This adds a helper test string. This is now only used in a few tests,
but the intention is to use this in more tests for basic_string.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D145257