Summary of Changes

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This pull request resolves an issue in the Spanner client library's ISO8601 interval parsing mechanism. It refactors the parsing logic to correctly handle the initial prefixes and the transition from date to time components, specifically by introducing a flag to manage the 'T' separator. This change prevents a subtle bug related to comparing iterators from different containers, enhancing the robustness and correctness of interval string interpretation.

Highlights

• Refactored ISO8601 Interval Parsing: The logic for consuming the optional sign (+/-) and the mandatory 'P' prefix for ISO8601 intervals has been moved to the beginning of the ParseISO8601Interval function, improving clarity and ensuring correct initial parsing.
• Introduced seen_T Flag: A new boolean flag, seen_T, was added to accurately track whether the 'T' separator (indicating the start of time components) has been encountered. This prevents redundant processing of the 'T' prefix.
• Prevented Iterator Comparison Issue: The changes address a potential bug where iterators from different unit factory containers (kISO8601DateUnitFactories and kISO8601TimeUnitFactories) could have been implicitly compared. The refactoring ensures that iterators are only compared within their respective valid ranges, thus avoiding undefined behavior.

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