[DDC] Don't use CFE constructor tearoff lowering and instead use the same lowering as static method tearoffs

[srujzs]

Currently, the CFE lowers tearoffs for constructors as static methods on the relevant class. DDC then forwards a call to this static method in order to fetch the tearoff. While this is useful, this means extra code size for every constructor e.g.

    static ['_#fact#tearOff']() {
      return library_name.ClassName.fact();
    }

and the calling code:

    get C0() {
      return C[0] = dart.fn(library_name.ClassName['_#fact#tearOff'], T$.VoidToClassName());
    },

C0 here is already a tear-off, so we should just leverage that instead of this dual indirection. We already do that with static methods:

    get C3() {
      return C[3] = dart.fn(library_name.ClassName.staticMethod, T$.VoidToString());
    },

So, instead of using the synthetic static methods for constructor tear-offs, we can just return a reference to the constructor instead e.g.

return C[0] = dart.fn(library_name.ClassName.fact, T.VoidToClassName());

[srujzs]

cc @nshahan