Fix sealing of types that were not concretely instantiated #73218
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Fixes #73076.
Because of the generic interface method use, the compiler didn't generate a concrete instantiation of one of the generic types anywhere and we were incorrectly sealing it, causing a bad devirtualization.
The fix is to look at template type instantiations too.
I'm also adding an unrelated regression test for scenarios I fixed in #64497. We could push the regression test through that PR but I've only edited the branch through GitHub UI and this is easier...
Cc @dotnet/ilc-contrib