Callee can be incorrectly overridden when it's captured

[Saam Barati]

Consider these two programs:
"(function foo() { foo = 20; print(foo); })();"
and
"(function foo() { let x = function() { return foo; }; foo = 20; print(foo) })();"
They have different behaviors.
The latter incorrectly assigns 20 to foo.
I think we can probably make this Just Work with a FunctionNameScope.

[Saam Barati]

Created attachment 259805 [details]
patch

[Saam Barati]

landed in:
http://trac.webkit.org/changeset/188926