8386: Avoid O(n²) when constructing AttrSourceMap r=jonas-schievink a=jonas-schievink Brings https://github.com/rust-analyzer/rust-analyzer/issues/8377 down to 2.52s on my machine. Not quite back to where it was before, so I'll leave that issue open for now. bors r+ Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>