[GN] Allow Xcode source indexing of non default files. BUG= Review-Url: https://codereview.chromium.org/2086463002 Cr-Original-Commit-Position: refs/heads/master@{#400919} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 7b90a1de5ac82360dcffce82744d6de866cb73ba