[Frontend] Avoid doing whole-module work under primary-file typecheck (#27153)

...a situation we get into with indexing. The way Xcode generates
indexing invocations is to take a build command and add additional
flags to it; in order for the Driver to produce a single frontend
command from /that/, it currently plans as if it's going to do a
whole-module -typecheck and then turns around and uses -primary-file
anyway. This is questionable practice, to be sure...

...but meanwhile, let's not crash by trying to access declarations
that haven't been type-checked yet.

rdar://problem/53117124

Author: jrose-apple

lib/FrontendTool/FrontendTool.cpp

@@ -1122,10 +1122,12 @@ static bool performCompile(CompilerInstance &Instance,
   if (Action == FrontendOptions::ActionType::Typecheck) {
     if (emitIndexData(Invocation, Instance))
       return true;
-    if (emitAnyWholeModulePostTypeCheckSupplementaryOutputs(Instance,
-                                                            Invocation,
-                                                            moduleIsPublic)) {
-      return true;
+    if (opts.InputsAndOutputs.isWholeModule()) {
+      if (emitAnyWholeModulePostTypeCheckSupplementaryOutputs(Instance,
+                                                              Invocation,
+                                                              moduleIsPublic)) {
+        return true;
+      }
     }
     return false;
   }

test/Index/Store/driver-index-with-auxiliary-outputs.swift

@@ -0,0 +1,35 @@
+// RUN: %empty-directory(%t)
+
+// This test is very deliberately *not* indexing the current file; we need to
+// make sure the frontend job doesn't try to emit the auxiliary outputs based
+// on the non-indexed files. (This is how Xcode currently constructs -index-file
+// invocations: take a normal build command and add extra arguments to it.)
+// RUN: %target-build-swift -index-file -index-file-path %S/Inputs/SwiftModuleA.swift %S/Inputs/SwiftModuleA.swift %s -index-store-path %t/idx -module-name driver_index -emit-objc-header-path %t/out.h -emit-module-interface-path %t/out.swiftinterface
+
+// RUN: test ! -f %t/out.h
+// RUN: test ! -f %t/out.swiftinterface
+// RUN: c-index-test core -print-unit %t/idx | %FileCheck %s
+
+// CHECK-LABEL: module-name: driver_index
+// CHECK: DEPEND START
+// CHECK-NOT: Record |
+// CHECK: Record | user | {{.+}}SwiftModuleA.swift
+// CHECK-NOT: Record |
+// CHECK: DEPEND END
+
+#if _runtime(_ObjC)
+
+// Do a stronger test here involving checking @objc
+import ObjectiveC
+
+public class PossiblyObjC: NSObject {
+  @objc public init(x: Int) {}
+}
+
+#else // _runtime(_ObjC)
+
+public class Boring {
+  init()
+}
+
+#endif // _runtime(_ObjC)