Add better support for function type in TClassEdit::CleanType

In the CINT world, funcptr were void* (often with #ifdef hacks), but in
the Cling world, they seem to be handled properly.

That's great!

However, TClassEdit::CleanType munches the "function" part of the funcptr
types, so e.g. "double (*)()" becomes just "double". This is a problem as
the CleanType() part is called (eventually) by TClingMethodArgInfo to get
its type name. I.e., well before PyROOT can even see it.

The patch below works for me to leave the "()" bits well alone. I've also
checked ROOT/meta and did not see any breakage (although I doubt that it
test function pointers).

I need this to get my function callbacks to work (TF1 & friends), as their
callback ctor take a funcptr.

[From Wim]