ImportC can now automatically run the preprocessor

[Walter Bright]

On 5/15/2022 8:59 PM, max haughton wrote:
> One thing I'd also like to note is that once the C has been turned into the D 
> AST you actually can almost get it to emit D code that represents the C

"almost" is the operative word, here :-/

One of the motivations for ImportC (as opposed to C=>D translations that htod, 
dstep, and dpp do) is that the semantics can be an exact match to C.