Workaround for Issue 21119 - Disable -betterC for coverage builds

[MoonlightSentinel]

because -betterC apparently breaks coverage generation

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21119	normal	Code coverage checker is not instrumenting backend source files

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#11522"

[MoonlightSentinel]

Temporarily removing auto-merge to check if everything works as expected with CodeCov.

[MoonlightSentinel]

Also needed to update the source file paths s.t. the *.lst are named relative to the srcdirectory (which was already done for lexer and dmd).

[MoonlightSentinel]

CodeCov works now and reports a slight coverage increase:

[WalterBright]

Thank you! This is great.

[WalterBright]

@MoonlightSentinel @wilzbach -cov requires linking with druntime, and -betterC explicitly must not require druntime. This is not a bug in the backend, and this fix is not a workaround, it's a fix.