UPSTREAM PR #17738: common: use native MultiByteToWideChar

[loci-dev]

Mirrored from ggml-org/llama.cpp#17738

std::codecvt_utf8<wchar_t> is deprecated and produces warnings:

common/common.cpp:792:31: warning: 'codecvt_utf8<wchar_t>' is deprecated [-Wdeprecated-declarations]
  792 |     std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
      |

Please, review carefully as I’ve used Gemini to generate utf8_to_wstring() and I'm absolutely not a Windows dev.

[loci-review]

Explore the complete analysis inside the Version Insights

Performance Analysis Summary: PR #420

Analysis Overview

This PR replaces deprecated std::codecvt_utf8 with native Windows API MultiByteToWideChar in fs_create_directory_with_parents function within common/common.cpp. The change affects filesystem initialization code, not inference or tokenization paths.

Performance Impact Assessment

Condition 1 applies: No meaningful performance changes detected.

The modified function handles directory creation during initialization and is not part of the inference or tokenization pipeline. Performance metrics for critical functions (common_token_to_piece, common_detokenize) show 5-11 ns improvements unrelated to this PR. No inference functions (llama_decode, llama_encode, llama_tokenize) were modified. Power consumption changes across all binaries are negligible (<0.008%). The code change eliminates compiler warnings without impacting tokens per second.