[clang] [clang-scan-deps] Fix "unterminated conditional directive" bug (PR #146645)

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> The logic added in #143950 also handles line splices with 0 trailing whitespace characters, so this code is now redundant. (When removing it, the test run fine too.)
> 
> Should removing this code be a separate PR?

Done

https://github.com/llvm/llvm-project/pull/146645