[clang] [clang-scan-deps] Ignore import/include directives with missing filenames (PR #99520)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> This is similar feedback that I gave for #97654, could you test the change with a unit test in `DependencyDirectivesScannerTest.cpp` instead? It's much simpler to set up and execution-wise it's orders of magnitude more lightweight than a whole new lit test.
> 
> For reference see the test change in that PR.

I switched to the unit test, though I didn't figure out how to capture the same sort of round-trip testing as with the lit test. 
It seems expected that the wrapper function `minimizeSourceToDependencyDirectives` returns all directives without actually evaluating them so it would always fail on an invalid one, even if it's macro guarded.

https://github.com/llvm/llvm-project/pull/99520