[clang-format] Assert Line->First. NFC.

Cf. scan-build reports:
* https://llvm.org/reports/scan-build/report-AffectedRangeManager.cpp-nonPPLineAffected-34-16c04b.html#EndPath
* https://llvm.org/reports/scan-build/report-SortJavaScriptImports.cpp-parseModuleReferences-34-96a7f8.html#EndPath
* https://llvm.org/reports/scan-build/report-TokenAnnotator.cpp-setCommentLineLevels-26-77bdba.html#EndPath
* https://llvm.org/reports/scan-build/report-AffectedRangeManager.cpp-nonPPLineAffected-31-714434.html#EndPath
* https://llvm.org/reports/scan-build/report-TokenAnnotator.cpp-setCommentLineLevels-16-bd39d0.html#EndPath
* https://llvm.org/reports/scan-build/report-UnwrappedLineFormatter.cpp-format-90-668b2d.html#EndPath