Add ImportC compiler to dmd

[Walter Bright]

On 5/12/2021 11:56 PM, Araq wrote:
> Sequence points, volatile, shared memory model, undefined behavior, obscure 
> aliasing rules... C is small and well understood indeed -- when you chose to 
> ignore all these things that happen to contradict your narrative.

Essentially none of that matters except sequence points for ImportC. And D 
follows the same sequence point rules.