Make DMD emit C++ .h files same as .di files

[Manu]

On Mon, Feb 25, 2019 at 2:25 AM Jacob Carlborg via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
>
> On 2019-02-25 02:04, Manu wrote:
>
> > Why wouldn't you do it in the same pass as the .di output?
>
> * Separation of concerns
> * Simplifying the compiler ("simplifying" is not the correct
> description, rather avoid making the compiler more complex)
>
> I think the .di generation should be a separate tool as well.

Compile times already suck pretty hard, I feel like it's a very
valuable feature that DMD can emit .di from the same compile pass.
It's already done all the work... why repeat that build cost a second
time for every source file?