GDScript: Fix missing warning for shadowing of built-in types

[vonagam]

func _ready():
  var Array := 3 # before: ok, after: warning

Extracted from #74618.

[YuriSizov]

Thanks!

PS. Now that it returns nothing, the name of GDScriptAnalyzer::is_shadowing is not great. It should probably be something like check_if_shadowing or test_shadowing, or some version of these.