[ASan][libc++] Annotating `std::basic_string` with all allocators (#75845)

This commit turns on ASan annotations in `std::basic_string` for all
allocators by default.

Originally suggested here: https://reviews.llvm.org/D146214

String annotations added here:
https://github.com/llvm/llvm-project/pull/72677

This commit is part of our efforts to support container annotations with
(almost) every allocator. Annotating `std::basic_string` with default
allocator is implemented in
https://github.com/llvm/llvm-project/pull/72677.

Additionally it removes `__begin != nullptr` because `data()` should
never return a nullptr.

Support in ASan API exists since
https://github.com/llvm/llvm-project/commit/1c5ad6d2c01294a0decde43a88e9c27d7437d157.
This patch removes the check in std::basic_string annotation member
function (__annotate_contiguous_container) to support different
allocators.

You can turn off annotations for a specific allocator based on changes
from
https://github.com/llvm/llvm-project/commit/2fa1bec7a20bb23f2e6620085adb257dafaa3be0.

The motivation for a research and those changes was a bug, found by
Trail of Bits, in a real code where an out-of-bounds read could happen
as two strings were compared via a call to `std::equal` that took
`iter1_begin`, `iter1_end`, `iter2_begin` iterators (with a custom
comparison function). When object `iter1` was longer than `iter2`, read
out-of-bounds on `iter2` could happen. Container sanitization would
detect it.

If you have any questions, please email:
- advenam.tacet@trailofbits.com
- disconnect3d@trailofbits.com