[Issue 22611] ImportC: Need better error messages involving structs

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22611

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
Internally, the compiler does not keep track of which typedefs are pointing to
a struct tag, so it's kinda awkward to fix.

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