Fix #22813 - Removing __monitor from Object leads to errors

[dkorpel]

Closes #22813

Need to check whether LDC and GDC use target.classinfosize

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[thewilsonator]

LDC does not.

[thewilsonator]

GDC appears not to: https://github.com/search?q=repo%3AD-Programming-GDC%2Fgcc%20classinfosize&type=code

[thewilsonator]

need to account for monitor here as done in verifyStructSize?