Add clang-tools to aosp-master.

clang-tools is needed for tools based on clang / LLVM which should be in
sync with the toolchain's version.
Test: repo sync -j128 fetches clang-tools
Change-Id: Iaa22dd47ac679e8097be9c7b386767d2ca0e5305

default.xml

@@ -581,6 +581,7 @@
   <project path="prebuilts/build-tools" name="platform/prebuilts/build-tools" groups="pdk" clone-depth="1" />
   <project path="prebuilts/checkcolor" name="platform/prebuilts/checkcolor" groups="pdk" clone-depth="1" />
   <project path="prebuilts/checkstyle" name="platform/prebuilts/checkstyle" groups="pdk" clone-depth="1" />
+  <project path="prebuilts/clang-tools" name="platform/prebuilts/clang-tools" groups="pdk" 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" clone-depth="1" />
   <project path="prebuilts/devtools" name="platform/prebuilts/devtools" groups="pdk-fs" clone-depth="1" />