[clangd][QueryDriver] Dont check for existence of driver

Execute implementations already checks for permissions and existence
and returns relevant errors as necessary, so instead of printing our own errors,
we just print theirs.

This also fixes a case in windows where the driver might be missing the `.exe`
suffix. Previously, clangd would reject such a driver because sys::fs::exists is
strict, whereas the underlying Execute implementation would check with `.exe`
suffix too.

Fixes https://github.com/clangd/clangd/issues/93

Differential Revision: https://reviews.llvm.org/D102431