Add ImportC compiler to dmd

[Walter Bright]

On 5/10/2021 3:03 AM, Basile B. wrote:
> There are chances that the discussion will derivate on the topic of full 
> translation of C while the point is rather to translate headers i.e 
> **declarations** and only that. Like if it can handle most commonly used c 
> headers, let's say zlib, ssl, etc. it's ok. What people want is more like cairo 
> headers, llvm headers, x11 headers, etc. to be handled.

We *can* do a full translation of C with ImportC.