srcSpanStartLine/srcSpanEndLine panic on UnhelpfulLoc. They should not
really be exported by SrcLoc at all, but unfortunately they are used in
Lexer.x, which knows enough to avoid the panic.

However the call in RnEnv didn't know, and the panic was triggered 
by Template Haskell spliced code.  This patch fixes it by exporting
the predicate RnEnv wanted, namely isOneLineSpan.