[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.

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2007,8 +2007,13 @@ InterfaceSubContextDelegateImpl::runInSubCompilerInstance(StringRef moduleName,
   subInstance.getSourceMgr().setFileSystem(SM.getFileSystem());
 
   ForwardingDiagnosticConsumer FDC(*Diags);
-  if (!silenceErrors)
+  NullDiagnosticConsumer noopConsumer;
+  if (!silenceErrors) {
     subInstance.addDiagnosticConsumer(&FDC);
+  } else {
+    subInstance.addDiagnosticConsumer(&noopConsumer);
+  }
+
   std::string InstanceSetupError;
   if (subInstance.setup(subInvocation, InstanceSetupError)) {
     return std::make_error_code(std::errc::not_supported);