[Issue 21119] Code coverage checker is not instrumenting backend source files

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21119

--- Comment #4 from Walter Bright <bugzilla at digitalmars.com> ---
-cov requires linking with druntime, and -betterC explicitly must not require
druntime. This is not a bug in the backend.

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