[aheejin]

This updates libcxx from 22.1.8 to LLVM 23.1.3:
https://github.com/llvm/llvm-project/releases/tag/llvmorg-23.1.3

Additional changes:
(More detailed descriptions are in the commit messages)

• Update __config_site: 3bfcd35
  This adds missing variables from __config_site.in. We define all of these as 0 because we use musl.

• Define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER: 3e88dd8
  [libc++] Introduce implicit and explicit ABI annotations llvm/llvm-project#193045 introduced _LIBCPP_{BEGIN/END}_EXPLICIT_ABI_ANNOTATIONS, which defines a region within which a set of attributes are automatically applied. If there are no functions between the two macros, Clang gives a warning, but the PR also suppressed the warnings. And later [libc++] Only push/pop diagnostics for ABI annotations pragmas if not in a system header llvm/llvm-project#200338 removed those warning-suppressing pragmas when _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER was not defined in order to boost compilation time. So we need to define it in our libc++ build. A more detailed description is in the commit message.

• Build libc++ with C++26: 849973e
  LLVM 23 started to build libc++ with C++26 ([libc++] Build the library with C++26 llvm/llvm-project#181021). And [libc++] Implement P1885R12: <text_encoding> llvm/llvm-project#141312 added text_encoding, which does not build without C++26.

• Define _LIBCPP_AVAILABILITY_MINIMUM_HEADER_VERSION=23: b275b07
  [libc++] Introduce the notion of a minimum header version llvm/llvm-project#166074, in LLVM 22, introduced the notion of minimum header version: https://libcxx.llvm.org/VendorDocumentation.html#minimum-header-version
  With a specific minimum version, we don't support and include symbols in previous LLVM headers but not in the current headers anymore. Emscripten has used _LIBCPP_ABI_VERSION = 2, which means we don't allow linking
  with older versions of objects anyway. So setting _LIBCPP_AVAILABILITY_MINIMUM_HEADER_VERSION to 23 (the
  current LLVM release) allows us to remove old unused symbols from the archives.

• Handle the removal of unused transitive includes: bc698ec, 2604b81, a0900c0, 8a9e606, f05fba9
  [libc++] Drop transitive includes by default llvm/llvm-project#195509 removed unused transitive includes by default, causing undefined identifier errors in our code. This adds necessary includes to our libraries and tests. For port builds, because we can't modify their source code, define _LIBCPP_KEEP_TRANSITIVE_INCLUDES_LLVM23, which temporarily allows us to keep the transitive includes.

• Update mimalloc code size limit in test_malloc_size: 17eac83

[gemini-code-assist]

Code Review

This pull request performs a major update and refactoring of libc++ headers, including updating the library version to 23.1.3, reorganizing configuration and attribute macros, adding new utility headers like valid_range.h and capacity_aware_iterator.h, and modernizing various algorithms and iterators. The review feedback identifies critical issues in the newly introduced __capacity_aware_iterator, where the absolute value calculation for bounds checking can trigger undefined behavior via signed integer overflow when the offset is the minimum possible value. Additionally, a potential compilation failure was noted in ranges::swap_ranges when the two ranges have mismatched difference types, which can be resolved by explicitly specifying their common type in std::min.