Use source_location instead of eval(__FILE__,binding) in Binding#irb

e9e17cbc05 (enabling the warning by
default) caused a warning in test-spec:
```
/data/chkbuild/tmp/build/20190802T213005Z/ruby/spec/ruby/core/binding/irb_spec.rb
Binding#irb
- creates an IRB session with the binding in scope/data/chkbuild/tmp/build/20190802T213005Z/ruby/spec/ruby/core/binding/fixtures/irb.rb:3: warning: __FILE__ in eval may not return location in binding; use Binding#source_location instead
```
20190802T213005Z.log.html.gz

ref: [Bug #4352]

lib/irb.rb

@@ -805,7 +805,7 @@ class Binding
   #
   # See IRB@IRB+Usage for more information.
   def irb
-    IRB.setup(eval("__FILE__"), argv: [])
+    IRB.setup(source_location[0], argv: [])
     workspace = IRB::WorkSpace.new(self)
     STDOUT.print(workspace.code_around_binding)
     IRB::Irb.new(workspace).run(IRB.conf)