Use substTyUnchecked in TcMType.new_meta_tv_x
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.

olsner/ghc,sdiehl/ghc,mettekou/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,mettekou/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,mettekou/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc