[clang] [Clang] Fix crash when -header-include-filtering is not specified (PR #136232)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 commented:

The diagnostic is confusing - it tells the user they used an invalid combination of flags, but shows flags they did not use at all. I think we can improve on this and report when a flag is deduced/assumed/defaulted.

https://github.com/llvm/llvm-project/pull/136232