[Sema] Allow the use of SPI in API for SPI modules

When the whole module has an SPI distribution, SPI declarations can be
used in API.

rdar://75335462

lib/FrontendTool/FrontendTool.cpp

@@ -862,7 +862,7 @@ static bool emitAnyWholeModulePostTypeCheckSupplementaryOutputs(
   }
 
   if (opts.InputsAndOutputs.hasPrivateModuleInterfaceOutputPath()) {
-    // Copy the settings from the module interface
+    // Copy the settings from the module interface to add SPI printing.
     ModuleInterfaceOptions privOpts = Invocation.getModuleInterfaceOptions();
     privOpts.PrintSPIs = true;