[Lexer] Fix missing coverage line after #endif

Summary: bug reported here: https://bugs.llvm.org/show_bug.cgi?id=46660

Reviewers: vsk, efriedma, arphaman

Reviewed By: vsk

Subscribers: dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83514