4996: Correctly generate new struct field in file containing struct def r=matklad a=TimoFreiberg
WIP because the test doesn't pass.
Testing the fix by hand looked good, although quickfixes seem to not support setting the editor cursor yet, which i think we want for "generate missing defs from usage" fixes.
Co-authored-by: Timo Freiberg <timo.freiberg@gmail.com>