Remove srcSpanStartLine/srcSpanEndLine crash
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.