Fix 22954 - dtoh: Ignore member functions with C linkage

[MoonlightSentinel]

Those functions receive D mangling and hence aren't easily callable via
the header file. Note that C++ does the same for members in an
extern "C" aggregate (=> mangle as C++ symbol).
`

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22954	normal	Header generator emits extern(C) member functions

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#13915"

[dkorpel]

Does this also ignore extern(C) functions mixed in at global scope?
Edit: For clarity, I mean with a template mixin, not a string mixin.

[MoonlightSentinel]

No. I've added an example to the dshell test.