Add special handling for zero-length arrays to enhance PTA precision

[cs-cat]

Maintaining a points-to-set for components of a zero-length array is unnecessary. Use a mock object ZeroLengthArray to represent the zero-length array object instead.

Some microbenchmarks suggest improvements in both precision and performance.

benchmark result on dacapo-2006:eclipse

context sensitivity	var points-to (insens)	var points-to (sens)	array points-to
ci	2509,6346	N/A	65,1524
ci+patch	2496,6038 (-0.5%)	N/A	62,7517 (-3.7%)
1obj	1142,7739	8890,1119	21,2872
1obj+patch	1136,7029 (-0.5%)	8829,0413 (-0.7%)	20,8039 (-2.3%)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.94%. Comparing base (3f3b802) to head (c7237a2).

Files with missing lines	Patch %	Lines
...taie/analysis/pta/core/heap/AbstractHeapModel.java	91.66%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #140      +/-   ##
============================================
+ Coverage     71.91%   71.94%   +0.02%     
- Complexity     4354     4365      +11     
============================================
  Files           479      479              
  Lines         15856    15868      +12     
  Branches       2175     2179       +4     
============================================
+ Hits          11403    11416      +13     
- Misses         3571     3573       +2     
+ Partials        882      879       -3     

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[zhangt2333]

Very great optimization. Thanks for your work.

I've left some suggestions for improving the clarity of the code. Please give feedback. Finally, don't forget to rerun and update the "*-expected.txt" file.

[cs-cat]

Thanks, @zhangt2333 !
Here are changes since the first version:

• code: Factor out method isZeroSizedArrayAllocation to improve readability.
• code: Include array type in alloc of the mock object.
• tests: Use PTAAssert in ZeroSizedArray test.
• misc: Reword commit and PR. Attach benchmark results in PR descriptopn.

[cs-cat]

revision 2:

• ZeroSized -> ZeroLength
• Add this PR to changelog