Add linux/darwin projects for new Clang prebuilts.

Bug: 23396112
These remove the need for creating new projects every time the version
number changes.
Change-Id: Iaeb3fa9d9da089c7c720148e242bd0522e298975

default.xml

@@ -472,6 +472,8 @@
   <project path="prebuilts/android-emulator" name="platform/prebuilts/android-emulator" clone-depth="1" groups="pdk-fs" />
   <project path="prebuilts/clang/darwin-x86/host/3.6" name="platform/prebuilts/clang/darwin-x86/host/3.6" groups="pdk,darwin" />
   <project path="prebuilts/clang/linux-x86/host/3.6" name="platform/prebuilts/clang/linux-x86/host/3.6" groups="pdk,linux" />
+  <project path="prebuilts/clang/host/darwin-x86" name="platform/prebuilts/clang/host/darwin-x86" groups="pdk,darwin" />
+  <project path="prebuilts/clang/host/linux-x86" name="platform/prebuilts/clang/host/linux-x86" groups="pdk,linux" />
   <project path="prebuilts/devtools" name="platform/prebuilts/devtools" groups="pdk-fs" />
   <project path="prebuilts/eclipse" name="platform/prebuilts/eclipse" groups="pdk" />
   <project path="prebuilts/eclipse-build-deps" name="platform/prebuilts/eclipse-build-deps" groups="notdefault,eclipse" />