Implement ImplicitCallTearoff AstNode class in analyzer

[srawlins]

Given the following code:

class C {
  void call(int p) {}
}
class D {}
extension on D {
  void call(int p) {}
}
void foo(C c, D d) {
  void Function(int) f1 = c;
  void Function(String) f2 = c; // Correctly reports an error
  void Function(int) g1 = d;
  void Function(String) g2 = d; // Should report an error
}

The analyzer currently has a rudimentary mechanism for type-checking these assignments, which cannot properly check g2, a call tear-off of an extension method, call. To properly handle this case, as well as implicit type-instantiated call method tear-offs, the analyzer needs to insert tear-off nodes for implicit call tear-offs.

Needed for #47366, #46020. Also related: #32864, #45936

[srawlins]

Done