Discovered workaround to previous commit's issue

- See the last two commit messages for more context.
- It seems that one can import the generated bridging header and if it
  is imported, it must be imported before the generated Swift header.

Author: ncooke3

Fixtures/MixedTargets/BasicMixedTargets/Tests/BasicMixedTargetTests/ObjcBasicMixedTargetTestsViaGeneratedBridgingHeader.m

@@ -0,0 +1,26 @@
+#import <XCTest/XCTest.h>
+
+// IMPORTANT: The generated "bridging" header must be imported before the
+// generated interop header.
+#import "BasicMixedTarget/BasicMixedTarget.h"
+#import "BasicMixedTarget-Swift.h"
+
+@interface ObjcBasicMixedTargetTestsViaGeneratedBridgingHeader : XCTestCase
+
+@end
+
+@implementation ObjcBasicMixedTargetTestsViaGeneratedBridgingHeader
+
+- (void)testPublicSwiftAPI {
+    // Check that Objective-C compatible Swift API surface is exposed...
+    Engine *engine = [[Engine alloc] init];
+}
+
+- (void)testPublicObjcAPI {
+    // Check that Objective-C API surface is exposed...
+    OldCar *oldCar = [[OldCar alloc] init];
+    Driver *driver = [[Driver alloc] init];
+    CarPart *carPart = [[CarPart alloc] init];
+}
+
+@end

Fixtures/MixedTargets/BasicMixedTargets/Tests/MixedTargetWithCustomModuleMapTests/ObjcMixedTargetWithCustomModuleMapTestsViaGeneratedBridgingHeader.m

@@ -1,5 +1,7 @@
 #import <XCTest/XCTest.h>
 
+// IMPORTANT: The generated "bridging" header must be imported before the
+// generated interop header.
 #import "MixedTargetWithCustomModuleMap/MixedTargetWithCustomModuleMap.h"
 #import "MixedTargetWithCustomModuleMap-Swift.h"
 

Sources/Build/BuildPlan.swift

@@ -1496,10 +1496,6 @@ public final class MixedTargetBuildDescription {
             interopSupportDirectory = nil
         }
 
-        // TODO(ncooke3): Is there a way to remove the generated umbrella
-        // header when compiling individual clang files? This will reduce the
-        // misuse of importing the generated umbrella header.
-
         // Clients will later depend on the public header directory, and, if an
         // umbrella header was created, the header's root directory.
         self.publicHeaderPaths = interopSupportDirectory != nil ?