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.
If a module was first read using the adjacent swiftmodule and then
reloaded using the swiftinterface, we would do an up to date check on
the adjacent module but write out the unit using the swiftinterface.
This would cause the same modules to be indexed repeatedly for the first
invocation using a new SDK. On the next run we would instead raad the
swiftmodule from the cache and thus the out of date check would match
up.
The impact of this varies depending on the size of the module graph in
the initial compilation and the number of jobs started at the same time.
Each SDK dependency is re-indexed *and* reloaded, which is a drain on
both CPU and memory. Thus, if many jobs are initially started and
they're all going down this path, it can cause the system to run out of
memory very quickly.
Resolves rdar://103119964.
