[Frontend] Ensure constraint solving runs salvaging
When index-while-building is enabled, system modules are rebuilt from
their interface with diagnostics silenced so failures are not propagated
to the build. This is enabled via a local diagnostic engine that has no
consumers.
Typechecking uses the lack of consumers to add
`ConstraintSystemFlags::SuppressDiagnostics`, which controls whether
salvaging (and output of diagnostics) is run. There are cases today
where salvaging can find a correct solution though, so we should ensure
that it's always run.
This is a quick workaround for the indexing case - we should instead
always run salvaging, regardless of whether diagnostics are suppressed
or not.
Resolves rdar://117133297.
