Pass through --sysroot

[lalten]

clang-tidy supports --sysroot and in fact needs it to find header files in some cases.
For example if building with a hermetic compiler and libc++/libstdc++.

The flag was first ignored in c0d0793 but I couldn't find a reason as to why the removal was necessary.

[erenon]

Thanks! The reason it was ignored to be able to run clang tidy, with only gcc setup with sysroot. If you pass a gcc sysroot to clang, bad things happen (the commit message could have been better...).
Let's try this change and see if ppl complain.