Bug 20673 – Improve error message for opDollar in pointers

A very common beginner mistake: ``` void main () { int* ptr; assert(ptr[0 .. $] == [1, 2, 3]); } ``` This will yield the following message: ``` diagxxx.d(4): Error: undefined identifier __dollar ``` Not quite as informative as it could be. This could be a good starting task for someone wanting to touch DMD. The error is produced here: https://github.com/dlang/dmd/blob/631473e8746050bd743fe238ba343c079e0f25e0/src/dmd/expressionsem.d#L2721

Recent dmd does at least use `$`: slice.d(20): Error: undefined identifier `$`

THIS ISSUE HAS BEEN MOVED TO GITHUB https://github.com/dlang/dmd/issues/17946 DO NOT COMMENT HERE ANYMORE, NOBODY WILL SEE IT, THIS ISSUE HAS BEEN MOVED TO GITHUB