/prebuilts/clang/host/linux-x86 needs to always be present.

We keep some common device-specific files in the Linux prebuilts as
well, so it can't be Linux-specific (for those that prune their repo
trees on Darwin).

Change-Id: Ifd16509e9de79f6265e944ad409ec8ddf2dc8419
Reviewed-on: https://googleplex-android-review.git.corp.google.com/863382
Reviewed-by: Dan Albert <danalbert@google.com>
Presubmit-Verified: TreeHugger Robot <android-build-prod@system.gserviceaccount.com>
Reviewed-by: Bill Yi <byi@google.com>
(cherry-picked from commit 91d5393e9b3c05ee64501bd41aa3cd399d359903)

default.xml

@@ -457,7 +457,7 @@
   <project path="prebuilts/clang/darwin-x86/host/3.6" name="platform/prebuilts/clang/darwin-x86/host/3.6" groups="pdk,darwin" clone-depth="1" />
   <project path="prebuilts/clang/linux-x86/host/3.6" name="platform/prebuilts/clang/linux-x86/host/3.6" groups="pdk,linux" clone-depth="1" />
   <project path="prebuilts/clang/host/darwin-x86" name="platform/prebuilts/clang/host/darwin-x86" groups="pdk,darwin" clone-depth="1" />
-  <project path="prebuilts/clang/host/linux-x86" name="platform/prebuilts/clang/host/linux-x86" groups="pdk,linux" clone-depth="1" />
+  <project path="prebuilts/clang/host/linux-x86" name="platform/prebuilts/clang/host/linux-x86" groups="pdk" clone-depth="1" />
   <project path="prebuilts/devtools" name="platform/prebuilts/devtools" groups="pdk-fs" clone-depth="1" />
   <project path="prebuilts/eclipse" name="platform/prebuilts/eclipse" groups="pdk" clone-depth="1" />
   <project path="prebuilts/eclipse-build-deps" name="platform/prebuilts/eclipse-build-deps" groups="notdefault,eclipse" clone-depth="1" />