fix intra-link resolution spans in block comments
This commit improves the calculation of code spans for intra-doc
resolution failures. All sugared doc comments should now have the
correct spans, including those where the comment is longer than the
docs.
It also fixes an issue where the spans were calculated incorrectly for
certain unsugared doc comments. The diagnostic will now always use the
span of the attributes, as originally intended.
Fixes#55964.
r? @QuietMisdreavus
a64cdec1b4