Fix Issue 12344 - .di generation doesn't include contracts in interfaces

[dkorpel]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	12344	normal	.di generation doesn't include contracts in interfaces

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#13943"

[dkorpel]

I need to figure out what's the difference between header1.di and header1i.di and whether it's right that contracts are not emitted in the latter.

[dkorpel]

Turns out -inline sets global.params.hdrStripPlainFunctions to false, and bodyToBuffer doesn't output contracts for bodyless functions ever. I changed it so it does.