Use substTyUnchecked in TcMType.new_meta_tv_x
Sadly, one of the indirect callers of this function doesn't yet enforce the in-scope set. It's in the TcHsType.tc_infer_args swamp, so I'm not going to wade in there today. The assertion tripped when investigating Trac #12785; but this patch does NOT fix the actual bug reported there.
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