fix(llcppsymg): skip generating variadic function as a receiver method

[MeteorsLiu]

A part of #510

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.14%. Comparing base (f7fa27f) to head (f30ca47).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #513   +/-   ##
=======================================
  Coverage   84.14%   84.14%           
=======================================
  Files          27       27           
  Lines        2738     2738           
=======================================
  Hits         2304     2304           
  Misses        391      391           
  Partials       43       43           

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