NLL: errors from free variables should highlight their span, not span of their lambda expression #51170
Description
NLL has regressed some test cases in that it will highlight the whole span of a lambda expression (i.e. the |args, ... | { body ... }) rather than focus in on the occurrence(s) of the relevant free variable(s) within that lambda expression.
One instances of this is:
compare that output with that from the old AST borrowck:
(In addition, AST borrowck is noting that such a capture of a free variable is occurring, and highlights the parameter list saying "capture occurs here"; but this additional detail seems like very low priority to fix, if we want to fix it at all.)
