implement AST representation for function types with and without a prototype.

This lets us pretty print stuff like this:

void foo() {
  int X;
  X = sizeof(void (*(*)())());
  X = sizeof(int(*)(int, float, ...));
  X = sizeof(void (*(int arga, void (*argb)(double Y)))(void* Z));

as:

  X = sizeof(void (*(*)())())
  X = sizeof(int (*)(int, float, ...))
  X = sizeof(void (*(int, void (*)(double)))(void *))

Ah the wonders of 'modern' C syntax!

llvm-svn: 39232