Add ImportC compiler to dmd

[Walter Bright]

On 5/12/2021 6:08 AM, deadalnix wrote:
>  From the PR
>> WalterBright added the Trivial label 3 days ago
> 
> Found it funny.
> 
> C is not a fast moving target, so this is probably worth doing.

While adding the Trivial label was indeed my little joke, it's not entirely 
wrong. C is a small and very well understood language, so implementing a 
compiler for it doesn't require much cleverness. I've also gone around the horn 
implementing a C compiler before, and learned to take the canal this time.