When deducing the return type of defaulted three-way comparison, instead
of asserting in the case of unsupported builtin types, just diagnose it.
For now, we add a test case for function pointers which will produce
slightly incorrect diagnostics. In future work, we will stop adding
function pointers to the candidate set and get the correct diagnostics.
Signed-off-by: Matheus Izvekov <email@example.com>
Depends on D103760