[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/ModuleInterfaceBuilder.cpp

@@ -329,12 +329,14 @@ bool ImplicitModuleInterfaceBuilder::buildSwiftModuleInternal(
       llvm::RestorePrettyStackState(savedInnerPrettyStackState);
     };
 
+    NullDiagnosticConsumer noopConsumer;
     llvm::Optional<DiagnosticEngine> localDiags;
     DiagnosticEngine *rebuildDiags = diags;
     if (silenceInterfaceDiagnostics) {
       // To silence diagnostics, use a local temporary engine.
       localDiags.emplace(sourceMgr);
       rebuildDiags = &*localDiags;
+      rebuildDiags->addConsumer(noopConsumer);
     }
 
     SubError = (bool)subASTDelegate.runInSubCompilerInstance(