fix issue 18536 - Bad stack traces when building with -m32mscoff

[rainers]

dmdDecodeString is harmful for non-mangled names, disable it for anything but OMF

[dlang-bot]

Thanks for your pull request, @rainers!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18536	normal	Bad stack traces when building with -m32mscoff

[wilzbach]

@rainers is there a chance to add a test for this, s.t. it doesn't regress?
Maybe something similar to https://github.com/dlang/druntime/blob/master/test/exceptions/Makefile, but for Windows?

[rainers]

It would have been caught if we'd run the dmd test suite for -m32mscoff, too. Maybe AppVeyor should be switched to that platform instead of Win64 which is already covered by the auto tester.

[wilzbach]

It would have been caught if we'd run the dmd test suite for -m32mscoff, too. Maybe AppVeyor should be switched to that platform instead of Win64 which is already covered by the auto tester.

Can't we run both? At the the average build time is ~18 minutes which isn't great, but still has some leeway for more.