[SAoC 2021] Replace druntime Hooks with Templates: Milestone 1, Week 1

[Teodor Dutu]

Hi,

My project wants to replace dmd hooks to druntime functions such 
as `_d_arrayctor`, `_d_arrayappendT` and `_d_arraycatT` with 
templates. This will make lowerings to these functions faster at 
runtime, as their templated versions will not require all the 
indirection used by the hooks. Speaking of indirection, at the 
end of this project, the `TypeInfo` class will be removed as all 
its information will be used at compile time, when instantiating 
the aforementioned templates. As a result, the maintainability of 
the compiler will be improved as lowerings will now take place in 
the semantic analysis phase, as opposed to the IR phase (where 
the hooks are currently present).

Dan Printzell worked on this in the past, as part of his [GSoC 
2019 project](https://github.com/Vild/GSOC2019). I'll be picking 
up the work that he left and then I'll continue with the 
remaining hooks.

During my first week, I worked on finishing Dan's 
[PR](https://github.com/dlang/dmd/pull/10102) for the 
`_d_array{,set}ctor` functions:
- My mentors (Eduard Stăniloiu and Răzvan Nițu) and I fixed some 
of [Dan's 
logic](https://github.com/dlang/dmd/blob/e7c81c3021f5948a2a743c0926969f5370178f28/src/dmd/expressionsem.d#L9008-L9016) for choosing when to perform the lowering or not. We skipped lowering in expressionsem.d for snippets like `S[3] arr = [S(1), S(2), S(3)];`, as e2ir.d already handles such cases more elegantly.
- The tests that are currently failing are runnable/testassign.d 
and compilable/interpret3.d. For example, runnable/testassign.d 
fails because in expressions like `funcVal(a = b)` the temporary 
variable used as the actual argument for `funcVal` is not lowered 
to use `_d_arraysetctor`, when it actually should. I am still 
working on fixing it.

Thanks,
Teodor