Fix resource bundle accessor issue for Clang sources

- I noticed an issue when trying to access a Clang package's resources
  when testing the target from the SwiftPM CLI. In short, the resource
  bundle is placed in the same place when building a Clang-only or
  Swift-only package, but the generated resource_bundle_accessor.m's logic
  tries to create an NSBundle for a path that DNE. The difference becomes
  clear when comparing the logic to the resource_bundle_accessor.swift
  that is generated from Swift-only packages with resources.

Author: ncooke3

Sources/Build/BuildPlan.swift

@@ -554,7 +554,13 @@ public final class ClangTargetBuildDescription {
 
         NSBundle* \(target.c99name)_SWIFTPM_MODULE_BUNDLE() {
             NSURL *bundleURL = [[[NSBundle mainBundle] bundleURL] URLByAppendingPathComponent:@"\(bundleBasename)"];
-            return [NSBundle bundleWithURL:bundleURL];
+
+            NSBundle *preferredBundle = [NSBundle bundleWithURL:bundleURL];
+            if (preferredBundle == nil) {
+              return [NSBundle bundleWithPath:@"\(bundlePath.pathString)"];
+            }
+
+            return preferredBundle;
         }
         """