This should improve the time it takes to run the test suite a bit. Right
now there are only a handful of headers in the modulemap because we're
missing a lot of includes in the tests. New headers should be added
there from the start, and we should fill up the modulemap over time
until it contains all the test support headers.

An LLVM 19-aligned libc++ was released with macOS 15.4 (and corresponding OSes),
and LLVM-20 aligned with macOS 26.0. This patch adds availability markup to
reflect that.

This patch contains several changes to `deallocate_size.pass.cpp`:
1. `static_cast`-ing some parameters to `size_t` to avoid narrowing.
2. Changing the type of loop variable `i` to `unsigned int` avoid
signedness mismatch with the constructor parameter.
3. Separately counting allocations and deallocations in variables
`allocated_` and `deallocated_`, and changing their type to `uint64_t`.
4. Avoiding `assert`-ing count of allocations when a `basic_string` is
allocated, just `assert`-ing after destruction instead.

Verify that the operation passed to resize_and_overwrite returns an
integer-like type, matching the behavior of other standard library
implementations like GCC's libstdc++

Fixes #160577

Apple M4:
```
Benchmark                                                      Baseline    Candidate    Difference    % Difference
-----------------------------------------------------------  ----------  -----------  ------------  --------------
BM_string_literal/1024                                            16.99        16.79         -0.21           -1.21
BM_string_literal/128                                              3.44         3.34         -0.10           -2.88
BM_string_literal/16                                               1.80         1.69         -0.11           -5.93
BM_string_literal/2048                                            38.44        38.38         -0.07           -0.17
BM_string_literal/256                                              5.77         5.65         -0.12           -2.02
BM_string_literal/32                                               2.03         1.92         -0.11           -5.44
BM_string_literal/4096                                            73.92        73.74         -0.18           -0.25
BM_string_literal/512                                              9.49         9.41         -0.08           -0.84
BM_string_literal/64                                               2.59         2.45         -0.14           -5.38
BM_string_literal/8                                                1.79         1.69         -0.11           -5.90
BM_string_literal/8192                                           132.09       131.81         -0.28           -0.21
```

Implements [P3044R2](https://wg21.link/P3044R2)

Note: `substr.pass.cpp` is refactored to accommodate the test of
`basic_string_view`'s `subview` which is an alias of `substr` without
changing the test cases.

Closes #148140

# References

- https://github.com/cplusplus/draft/pull/7975
- https://wg21.link/string.substr
- https://wg21.link/string.view.ops

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>
Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

This makes the test a bit easier to understand.

We've built up quite a few links directly to github within the code
base. We should instead use `llvm.org/PR<issue-number>` to link to bugs,
since that is resilient to the bug tracker changing in the future. This
is especially relevant for tests linking to bugs, since they will
probably be there for decades to come. A nice side effect is that these
links are significantly shorter than the GH links, making them much less
of an eyesore.

This patch also replaces a few links that linked to the old bugzilla
instance on llvm.org.

Currently, when the string shrink into the SSO buffer, the `__rep_.__s`
member isn't activated before the `traits_type::copy` call
yet, so internal `__builtin_memmove` call writing to the buffer causes
constant evaluation failure. The existing test coverage seems a bit
defective and doesn't cover this case - `shrink_to_fit` is called on the
copy of string after erasure, not the original string object.

This PR reorders the `__set_short_size` call, which starts the lifetime
of the SSO buffer, before the copy operation. Test coverage is achieved
by calling `shrink_to_fit` on the original erased string.