Make sure that we check for type errors strictly in a Template Haskell splice, even if -fdefer-type-errors is on

We're going to run this code, so there's no point in deferring type
errors.  Worse, TcSplice uses unsafeCoerce (for vevy good reasons),
so splicing a type into a place where an expression is expected would
yield a seg-fault if we plough on regardless.

Fixes Trac #7276