[Frontend] Set "failed to load" for failed partial module

If a partial module fails to load, the main module
may be left without files, so let's set the failed
to load bit.

Author: hamishknight

lib/Frontend/Frontend.cpp

@@ -761,8 +761,12 @@ void CompilerInstance::performSemaUpTo(SourceFile::ASTStage_t LimitStage,
   // If we aren't in a parse-only context, load the remaining serialized inputs
   // and resolve implicit imports.
   if (LimitStage > SourceFile::Unprocessed &&
-      loadPartialModulesAndImplicitImports())
+      loadPartialModulesAndImplicitImports()) {
+    // If we failed to load a partial module, mark the main module as having
+    // "failed to load", as it may contain no files.
+    mainModule->setFailedToLoad();
     return;
+  }
 
   // Then parse all the input files.
   // FIXME: This is the only demand point for InputSourceCodeBufferIDs. We

test/Serialization/failed-partial-module.swift

@@ -0,0 +1,7 @@
+// RUN: %empty-directory(%t)
+
+// Make sure we don't crash if we fail to load a partial module.
+// RUN: %target-swift-frontend -emit-module -primary-file %s -module-name A -o %t/a.partial.swiftmodule
+// RUN: not %target-swift-frontend -emit-module -merge-modules %t/a.partial.swiftmodule -module-name Unrelated 2>&1 | %FileCheck %s
+
+// CHECK: error: cannot load module 'A' as 'Unrelated'