Copy clang headers to reduce dependencies

This commit copies clang headers to reduce dependencies between
repositories.  Binaries under `prebuilts/clang-tools` should always uses
the clang headers distributed with themselves.

Bug: 111579848
Test: `clang-headers` is in `build-prebuilts.zip`.
Change-Id: Iaa233fe3ff628a23878163bef2dee19af978c464
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index bcdd8e7..20e5d33 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -89,6 +89,9 @@
 cp "${binaries[@]}" "${SOONG_OUT}/dist/bin/"
 cp -R "${SOONG_HOST_OUT}/lib"* "${SOONG_OUT}/dist/"
 
+# Copy clang headers
+cp -R "external/clang/lib/Headers" "${SOONG_OUT}/dist/clang-headers"
+
 # Package binaries and shared libs
 (
     cd "${SOONG_OUT}/dist"