Fix(#1765): Use thunk function as placeholder when it can't be resolved

[FXTi]

This commit use thunk function itself as an acceptable choice when actual callee can not be resolved, which could be caused by empty function body of itself.

Issue: #1765

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.00%. Comparing base (c711dc3) to head (36953c3).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
svf-llvm/lib/CHGBuilder.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1766   +/-   ##
=======================================
  Coverage   64.00%   64.00%           
=======================================
  Files         244      244           
  Lines       24454    24454           
  Branches     4611     4610    -1     
=======================================
  Hits        15653    15653           
  Misses       8801     8801           

Files with missing lines	Coverage Δ
svf-llvm/lib/CHGBuilder.cpp	83.51% <0.00%> (ø)

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