[slang] Fix bug with SignalEventControl iff clause emitting.

[likeamahoney]

There is copypasta bug with SignalEventControl iffCondition emitting during it's AST constructing. Right hand of iff was constructed from left hand part.

And I also added support for iffCondition serializing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.23%. Comparing base (04dedf0) to head (38fe37e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1019   +/-   ##
=======================================
  Coverage   94.22%   94.23%           
=======================================
  Files         191      191           
  Lines       47552    47554    +2     
=======================================
+ Hits        44807    44812    +5     
+ Misses       2745     2742    -3     

Files	Coverage Δ
source/ast/TimingControl.cpp	90.21% <100.00%> (+0.06%)	⬆️

... and 2 files with indirect coverage changes

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[MikePopoloski]

Nice, thanks for fixing.