Fix Bugzilla 20802 - Link failure with writefln

[tim-dlang]

The issue is caused by compiling druntime/phobos and the application with different flags. The template instance for __switch_errorT is not included in the build of druntime/phobos, because they are compiled with -release. DMD will also not include it in an application compiled without -release, because it assumes, that it is already in druntime. See comment https://issues.dlang.org/show_bug.cgi?id=20802#c3 by Spoov.

The template instance for __switch_errorT is now always instantiated in druntime, so it is part of the ABI of druntime.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20802	regression	[REG2.088.0] Link failure with writefln

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 "stable + dmd#16200"

[ibuclaw]

As a workaround the template instance for __switch_errorT is now always instantiated in druntime.

Why is this called a workaround? I think it'd be better to just say that this symbol is part of the druntime ABI (an ABI should be defined) and make it permanent.

[tim-dlang]

Why is this called a workaround? I think it'd be better to just say that this symbol is part of the druntime ABI (an ABI should be defined) and make it permanent.

I changed the commit text.