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

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

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 llvm#72677.

Support in ASan API exests since llvm@dd1b7b7.
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 llvm@2fa1bec.

This PR is a part of a series of patches extending AddressSanitizer C++ container overflow detection capabilities by adding annotations, similar to those existing in `std::vector` and `std::deque` collections. These enhancements empower ASan to effectively detect instances where the instrumented program attempts to access memory within a collection's internal allocation that remains unused. This includes cases where access occurs before or after the stored elements in `std::deque`, or between the `std::basic_string`'s size (including the null terminator) and capacity bounds.

The introduction of these annotations was spurred by a real-world software bug discovered by Trail of Bits, involving an out-of-bounds memory access during the comparison of two strings using the `std::equals` function. This function was taking iterators (`iter1_begin`, `iter1_end`, `iter2_begin`) to perform the comparison, using a custom comparison function. When the `iter1` object exceeded the length of `iter2`, an out-of-bounds read could occur on the `iter2` object. Container sanitization, upon enabling these annotations, would effectively identify and flag this potential vulnerability.

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

This commit addresses some comments from a code review:
- add size check,
- add coments with context to a test,
- add comments explaining what happens in the test,
- remove volatile,
- split an if to constexpr if and normal if.

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

Also refactor one line.