Add ImportC compiler to dmd

[Basile B.]

On Monday, 10 May 2021 at 09:31:58 UTC, Ola Fosheim Grostad wrote:
> Anyway, I suggested the approach Walter is taking here years 
> ago, but at this point there are more critical issues on the 
> table... This is a diversion from adressing the hard issues.

The changelog mentions [htod, dpp and 
DStep](https://github.com/dlang/dmd/pull/12507/files#diff-cdf7d0893f3c1cb898585c88423f77c6d7094b31425d18f29591809fdb928cabR37). The solution proposed should be as good as the union of the three.

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.