[Issue 22489] C header generation ignores custom mangling

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22489

moonlightsentinel at disroot.org changed:

           What    |Removed                     |Added
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                 CC|                            |moonlightsentinel at disroot.o
                   |                            |rg

--- Comment #1 from moonlightsentinel at disroot.org ---
`extern(C)` functions should just be exported by using the custom mangle as the
identifier. `extern(C++)` functions probably need to be ignored because C++
doesn't have something like `pragma(mangle)`

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