This allows the clang driver to know which tool is meant to be executed,
which allows the clang driver to load the right clang config files, and
allows clang to find colocated sysroots.
This makes sure that doing `clang-scan-deps -- <tool> ...` looks up
things in the same way as if one just would execute `<tool> ...`, when
`<tool>` isn't an absolute or relative path.
