core: Add source location fallback

[sargunv]

To support #4315, I added a fallback source location helper.

OpenHarmony's build environment lacks the <source_location> header. We previously had "A temporary shim for partial C++20 support" that patched the struct into the std namespace (which is undefined behavior), but even that wasn't sufficient as the header itself was included unconditionally.

Here I replace that shim with a custom mbgl::SourceLocation, which aliases to std::source_location when available and falls back to file/function/line otherwise.

Coded with Codex CLI / GPT-5.5 as part of #4315; see that PR for AI usage details.

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.0% -2.18Ki  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4318-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +51% +59.1Mi  +467% +27.9Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4318-compared-to-legacy.txt

[github-actions]

Benchmark Results ⚡

Benchmark                                                     Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                            -0.0065         -0.0061             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-4318-compared-to-main.txt

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-4318-compared-to-main.txt

[louwers]

Maybe we can get rid of it completely, let me ask.