When using clangd for cross-compiled projects, it's necessary to use the --query-driver flag so that clangd can extract the compiler's built-in header paths. However, there's no such flag for clangd-indexer so we're unable to build a working static index for these projects.
This patch adds a --query-driver flag to clangd-indexer for this scenario.
I saw some tests under clang-tools-extra/clangd/test/ but I think the cross-compilation case is a bit more complex to test. Let me know if you'd like me to look into this further.