[ModuleInterface] Error for out-of-date modules in the resource-dir (#24139)

If we're loading a .swiftmodule from the resource dir adjacent to the compiler,
defer to the serialized loader instead of falling back. This is mainly to
support development of Swift, where one might change the module format version
but forget to recompile the standard library. If that happens, don't fall back
and silently recompile the standard library -- instead, error like we used to.

rdar://49926152

Author: Harlan Haskins

lib/Frontend/ParseableInterfaceModuleLoader.cpp

@@ -839,6 +839,12 @@ class ParseableInterfaceModuleLoaderImpl {
     return scratch.str();
   }
 
+  bool isInResourceDir(StringRef path) {
+    StringRef resourceDir = ctx.SearchPathOpts.RuntimeLibraryPath;
+    if (resourceDir.empty()) return false;
+    return path.startswith(resourceDir);
+  }
+
   /// Finds the most appropriate .swiftmodule, whose dependencies are up to
   /// date, that we can load for the provided .swiftinterface file.
   llvm::ErrorOr<DiscoveredModule> discoverUpToDateModuleForInterface(
@@ -944,6 +950,21 @@ class ParseableInterfaceModuleLoaderImpl {
                                 << modulePath
                                 << "; deferring to serialized module loader\n");
         return std::make_error_code(std::errc::not_supported);
+      } else if (isInResourceDir(modulePath) &&
+                 loadMode == ModuleLoadingMode::PreferSerialized) {
+        // Special-case here: If we're loading a .swiftmodule from the resource
+        // dir adjacent to the compiler, defer to the serialized loader instead
+        // of falling back. This is mainly to support development of Swift,
+        // where one might change the module format version but forget to
+        // recompile the standard library. If that happens, don't fall back
+        // and silently recompile the standard library -- instead, error like
+        // we used to.
+        LLVM_DEBUG(llvm::dbgs() << "Found out-of-date module in the "
+                                   "resource-dir at "
+                                << modulePath
+                                << "; deferring to serialized module loader "
+                                   "to diagnose\n");
+        return std::make_error_code(std::errc::not_supported);
       } else {
         LLVM_DEBUG(llvm::dbgs() << "Found out-of-date module at "
                                 << modulePath << "\n");