Read a std::u8string from a JSON string and store it as one in every configuration (#6075)

* Read a std::u8string from a JSON string and store it as one in every configuration

In C++20, json(std::u8string) was stored as a JSON string only when
std::filesystem::path support was available (the to_json overload sat
inside the JSON_HAS_FILESYSTEM guard), and there was no matching
from_json, so get<std::u8string>() threw type_error.302 ("type must be
array, but is string"). Without std::filesystem, the same expression
still produced an array of code units (the 3.12.0 behavior).

Move the to_json overload out of the filesystem guard so that a
std::u8string is a string in every configuration, build the value
directly from the code units (this also works for a custom string_t
such as the tests' alt_string), and add a from_json overload that reads
a JSON string. The array of numbers written by 3.12.0 is still accepted;
other types throw type_error.302 ("type must be string or array").
std::u8string_view and char8_t arrays are unchanged.

The tests now run in C++20/23 (the file mentions JSON_HAS_CPP_20/23),
also without std::filesystem (test-conversions2_no_filesystem), and with
ordered_json and a custom string type. The new behavior is documented.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fill the std::u8string with resize and indexing instead of push_back

The push_back loop is not vectorized: reading a 1 kB string took about
2.2 us against 0.39 us with resize and direct element access (clang -O2).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Write the std::u8string from_json template header in the compact form

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Ignore libstdc++ char to char8_t sign change in the clang sanitizer job

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

cmake/clang_sanitizer_ignorelist.txt

@@ -9,6 +9,11 @@
 # __n2)`, which stores `__n1 - __n2` in a signed difference_type and clamps it to int. Any comparison with a shorter
 # view wraps that subtraction on purpose, and -fsanitize=integer reports it as implicit-integer-sign-change. json_view
 # compares keys as string views, so the same check is excluded for <string_view> as well.
+# std::filesystem::path::u8string() in libstdc++ 14 builds a std::u8string from the native char sequence, so
+# basic_string's range copy (_S_copy_chars) converts every char >= 0x80 to char8_t, which -fsanitize=integer reports as
+# implicit-integer-sign-change (test-conversions2_cpp20/cpp23, non-ASCII path). The conversion is intended and happens
+# inside the standard library, not in this library.
 [implicit-integer-sign-change]
 src:*/include/c++/*/format
 src:*/include/c++/*/string_view
+src:*/include/c++/*/bits/basic_string.h