Rustc of course already WF-checked the field types at the definition
site, but for error tainting of consts to work properly, there needs to
be an error emitted at the use site. Previously, with no use-site error,
we proceeded with CTFE and ran into ICEs since we are running code with
type errors.
Emitting use-site errors also brings struct-like constructors more in
line with fn-like constructors since they already emit use-site errors
for WF issues.
Note that the test output is currently *incorrect*. We should be
emitting an error at the use site too, not just at the definition. This
is partly for UI reasons, but mainly to fix a related ICE where a const
generic body is not tainted with an error since no usage error is
reported.
