Share object maps in value_sett::make_union when destination is empty

[tautschnig]

The object_mapt representation is reference counted precisely so that multiple owners can share one map, yet make_union built the union element by element even when the destination was empty. Value-set queries for a single symbol (via get_value_set_rec) arrive here with an empty destination, so each such query copied the symbol's entire object map, including one std::optional offset per element. Sharing the source map instead makes this constant time and removes the dominant allocation churn from value-set queries in symbolic execution.

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[Copilot]

Pull request overview

This PR improves pointer-analysis value-set performance by avoiding element-by-element copying in value_sett::make_union when the destination map is empty. It leverages the existing copy-on-write reference_counting<> representation to share the underlying object map in constant time, reducing allocation churn during frequent single-symbol value-set queries in symbolic execution.

Changes:

• Early-return when the source object map is empty.
• When the destination object map is empty, assign dest = src to share the reference-counted representation instead of inserting entries one-by-one.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.83%. Comparing base (f71fdad) to head (29eeb0d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9125   +/-   ##
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  Coverage    80.83%   80.83%           
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  Files         1715     1715           
  Lines       189948   189951    +3     
  Branches        73       73           
========================================
+ Hits        153540   153543    +3     
  Misses       36408    36408           

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