[PLAT-6395] Increase warning strictness

[nickdowell]

Goal

Increase the level of compiler diagnostics in the main Xcode project in order to detect warnings and potential bugs.

Changeset

Extra warning flags are configured in Bugsnag.xcconfig and apply at the project level. GCC_TREAT_WARNINGS_AS_ERRORS is disabled for unit test targets because they contain code that intentionally breaks the rules.

The warnings that required the most changes:

• -Wimplicit-atomic-properties required being explicit about atomicity of properties. Since most Bugsnag objects are not accessed from multiple threads simultaneously, properties that were previously implicitly atomic are now explicitly nonatomic.
• -Wnullable-to-nonnull-conversion required a large number of changes related to deserializing objects from JSON in particular. Previously there were cases where properties that were declared as nonnull could in fact be null and could have caused crashes when accessed from Swift. The nullability of properties has been kept the same to avoid breaking any client source code that depends on values being non-optional.
• Warnings related to signed/unsigned comparisons and implicit conversions between types required minor changes.

Testing

Tested via unit tests and full E2E test runs.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size increased by 4,376 bytes from 1,119,520 to 1,123,896

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