Fix name locations of FunDef and External nodes

[HertzDevil]

Errors for Def nodes are reported at their names:

macro f(x)
  {% x.foo %}
end

f(def foo; end)
#  5 | f(def foo; end)
#            ^--
# Error: undefined macro method 'Def#foo'

{% Array.methods[0].foo %}
#  145 | def initialize(size : Int, value : T)
#            ^---------
# Error: undefined macro method 'Def#foo'

Errors for both top-level and lib funs are not:

macro f(x)
  {% x.foo %}
end

f(fun foo; end)
#  5 | f(fun foo; end)
#        ^
# Error: undefined macro method 'FunDef#foo'

{% LibC.methods[0].foo %}
#  16 | fun dlclose(handle : Void*) : Int
#       ^------
# Error: undefined macro method 'External#foo'

This PR matches the latter's behavior with the former.