If root type of a key path has been determined to be a hole there
is no reason to delay the inference decision which should be a
failure because none of the components would be inferrable from
a placeholder root.
Produce a tailored diagnostic that omits a fully unresolved key path
type (`KeyPath<_, _>`) when key path without an explicit root type is
passed as an argument to non-keypath parameter type (i.e. `Int`).
`repairFailures` needs a special case when l-value conversion is
associated with a result type of subscript setter because otherwise
it falls through and treats result type as if it's an argument type,
which leads to crashes.
Resolves: rdar://84580119
