[Issue 23971] Provide clearer error message when trying to return a slice with C++ linkage

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23971

--- Comment #1 from Marcelo Silva Nascimento Mancini <msnmancini at hotmail.com> ---
Oh yea, the current error message is pretty bad:

`cannot return type const(char)[] because its linkage is extern(C++)`

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