If the enum has a raw value, `E` passed in to `BinaryExpr` is a `DeclRefExpr`, not a `DotSyntaxCallExpr`, so we crashed on `dyn_cast<DotSyntaxCallExpr>(E->getFn())->getFn()`.
Only look through `DotSyntaxCallExpr` if `E` is indeed a `DotSyntaxCallExpr`. While at it, fix a few other potential sources of nullptr crashes.
rdar://84707973
