Add weak __libcpp_verbose_abort definition to app on iOS

[AustinBenoit]

Description

Provide details of the change, and generalize the change in the PR title above.

On older iOS versions (iOS 15 and 16), the system libc++.1.dylib does not contain the std::__1::__libcpp_verbose_abort symbol. Since we build the Firebase C++ SDK binaries using a newer Xcode version (which references this symbol for standard library assertions), apps linking our static libraries will crash at startup on these older OS versions with a "Symbol not found" dynamic linker error. This was a change we had made to our test apps but we should include in to make the build process easier for all.

To resolve this:

• Added a weak definition of __libcpp_verbose_abort to app/src/app_ios.mm so it is packaged into libfirebase_app.a and statically resolved for any linking application.
• Removed the now-redundant temporary workaround definition from testing/sample_framework/src/app_framework.cc.

Fixes #1872

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Ran a local test in ios 16 and verified the symbol is loading correctly. Will run the integration test to verify everything continues to work.

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Type of Change

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• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[gemini-code-assist]

Code Review

This pull request moves the workaround for the Xcode 15+ / Swift 5.10+ Concurrency and C++ verbose abort crash on older iOS versions from the sample framework to the core iOS app implementation, and updates the release notes accordingly. Feedback suggests using C++ standard library headers instead of C headers, and using the _LIBCPP_ABI_NAMESPACE macro instead of hardcoding the __1 inline namespace to make the workaround more robust.