6751: Use the right `def_crate` for builtin macros r=jonas-schievink a=jonas-schievink
Fixes the incorrect macro resolution in https://github.com/rust-analyzer/rust-analyzer/issues/6716
No test, because diagnostics do not get remapped correctly for some reason. I've checked manually that this fixes the resolution errors.
bors r+
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>