[support] ParentIndexIterator: Add subscript and friend plus operators

[sbc100]

ParentIndexIterator declares using iterator_category = std::random_access_iterator_tag;, but did not provide the subscript operator (operator[]) or friend commutative addition (friend operator+(difference_type, const ParentIndexIterator&)).

When #8912 introduced std::sort(begin(), end()) on AndedConstraintSet (which inherits from inplace_vector), building binaryen failed on the Chromium emscripten-releases macOS builders with:

  error: type 'wasm::inplace_vector<...>::Iterator' does not provide a subscript operator

This failure did not show up on Binaryen's CI because libstdc++ implements std::sort using pointer/iterator addition and dereferencing (*(first + child)). In contrast, on macOS with -stdlib=libc++, libc++ implements std::sort (std::__sift_down) using __first[__child] and __first[__start], which requires operator[].

This commit adds decltype(auto) operator[] and friend operator+ to ParentIndexIterator to satisfy RandomAccessIterator requirements.

[sbc100]

Apparently we don't have any pure libc++ testing in binaryen CI:

1. Linux CI Jobs: All Ubuntu Linux builds in ci.yml (including build-clang, build-asan, build-tsan, build-ubsan, and build-noasserts) compile with clang or gcc using GNU's libstdc++ by default (/usr/include/c++/...). None pass -stdlib=libc++ or install libc++-dev.
2. macOS CI Job: The only CI job using libc++ is macos-14, which uses Apple Clang and Apple's system libc++ from Xcode. However, Apple's system libc++ is based on an older branch of LLVM libc++ where std::sort / std::__sift_down still used iterator addition and dereferencing (*(first + child)).

[kripken]

Thanks!

[sbc100]

I'm going to wait before landing to confirm that #8917 fails in CI (without this change)

[sbc100]

Sadly I can't repro with #8917 since it only repros is newer version of libc++