This changes Thread::Location::Backtrace#absolute_path to return
nil for methods/procs defined in eval. If the realpath of an iseq
is nil, that indicates it was defined in eval, in which case you
cannot use RubyVM::AbstractSyntaxTree.of.
Fixes [Bug #16983]
Co-authored-by: Koichi Sasada <ko1@atdot.net>