Add ImportC compiler to dmd

[Max Haughton]

On Sunday, 9 May 2021 at 20:57:06 UTC, Walter Bright wrote:
> https://github.com/dlang/dmd/pull/12507
>
> If you could add a C compiler to dmd with 3000 lines of code, 
> so C code could be imported directly? I would!

Things which can be said in C but not understood in D (like 
_Atomic) should probably fail to compile or compile to some 
core.compat.atomic type.