Add three regression test cases for cross-package interface private method calls:
1. _cmptest/issue1370_gotypes/ - Tests go/types.Object interface with Scope.Insert()
which calls the private setParent() method
2. _cmptest/issue1370_goast/ - Tests go/ast.Expr interface conversion with ast.Ident
which has a private exprNode() method
3. _cmptest/issue1370_geometry/ - Tests custom geometry.Shape interface with private
methods (validate, setID) across package boundaries
These tests verify the fix in ssa/abitype.go correctly sets the interface metadata's
PkgPath to the interface definition package instead of the compilation package,
preventing segmentation faults when calling private methods.
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Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>
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