[SAoC 2021] Replace druntime Hooks with Templates: Milestone 3, Week 4

[Teodor Dutu]

Hi,

I spent most of this week working on replacing `_d_delstruct` 
with a template. I first did it through [this 
PR](https://github.com/dlang/druntime/pull/3639) for druntime, 
followed by [this one](https://github.com/dlang/dmd/pull/13398) 
to change the lowering. Razvan [pointed out to 
me](https://github.com/dlang/dmd/pull/13398#discussion_r764625641) that I had made a mistake by not considering the case when the `-gc_profile` parameter was given. Hence, I wrapped `_d_delstruct` in a template and added a new hook called `_d_delstructTrace` as an alias to the already implemented `_d_delstruct` [here](https://github.com/dlang/druntime/pull/3644).

In the meantime, I resumed the work on the `_d_arrayappendcTX` 
and `_d_arrayappendT` hooks. I fixed my error from the previous 
week and now tests are running fine in dmd and druntime. There 
are still, however, a few errors when testing phobos. They are 
caused by the fact that `_d_arrayappendT` is declared `pure` (and 
needs to be so in order to be lowererd to from pure contexts), 
but calls `copyEmplace`, which, in those cases, is not pure 
because of the postblit it calls. I am still investigating and 
trying to narrow down the cases when impure postblits trigger 
this error, yet don't trigger errors when the origninal 
expression passes through semantic analysis.

Furthermore, when fixing the bug from last week, I had to 
determine which expressions `a` can be used unaltered in the 
lowering below, and which have to be saved in a temporary 
variable first.
```d
__ctfe ? a ~= x : _d_arrayappendcTX(a, 1), a[$ - 1] = x, a;
```
For now, the decision is oversimplified as it only checks for a 
few expression types such as variables or index expressions. 
Index expressions cannot be saved to temporaries because of [this 
bug](https://issues.dlang.org/show_bug.cgi?id=9023).
Saving `s["a"]` in a variable would trigger a CTFE error, because 
`s` is uninintialised.
However, if `a` from the lowering above is `foo()[i]`, then `a` 
is an index expression, but the function call can also have side 
effects, which makes it necessary to use a temporary variable 
here, as well. And what if `foo()` itself returns an 
uninitialised associative array? I'll have to give this logic 
some more thought over the course of next week.

Thanks,
Teodor