GROOVY-11616: STC: combine multiple type witnesses for type parameter

[eric-milles]

In simple cases, types can be merged to common super type.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.0013%. Comparing base (14fcb08) to head (50d035a).
⚠️ Report is 408 commits behind head on master.

Files with missing lines	Patch %	Lines
...roovy/transform/stc/StaticTypeCheckingSupport.java	80.0000%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2219        +/-   ##
==================================================
+ Coverage     68.9982%   69.0013%   +0.0031%     
- Complexity      29659      29661         +2     
==================================================
  Files            1423       1423                
  Lines          114319     114324         +5     
  Branches        19832      19835         +3     
==================================================
+ Hits            78878      78885         +7     
+ Misses          28811      28808         -3     
- Partials         6630       6631         +1     

Files with missing lines	Coverage Δ
...roovy/transform/stc/StaticTypeCheckingVisitor.java	87.4718% <ø> (ø)
...roovy/transform/stc/StaticTypeCheckingSupport.java	81.6622% <80.0000%> (-0.0075%)	⬇️

... and 4 files with indirect coverage changes

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