Make the location in TcLclEnv and CtLoc into a RealSrcSpan
Previously it was a SrcSpan, which can be an UnhelpulSrcSpan, but actually for TcLclEnv and CtLoc we always know it is a real source location, and it's good to make the types reflect that fact. There is a continuing slight awkwardness (not new with this patch) about what "file name" to use for GHCi code. Current we say "<interactive>" which seems just about OK.
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