[Issue 19579] `-HC` option to emit C/C++ headers

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19579

--- Comment #3 from Manu <turkeyman at gmail.com> ---
It would be disappointing for this to be a second pass by a separate tool
rather than just working it into the existing .di output flow.
Build times are always a problem, and running a compiler over each module twice
feels like a big waste of CI resources when the code is already digested and
completely resolved by the compiler, and there's already a path within the
compiler that does exactly this task.

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