Use clang-r399163b

libLLVM.so in the new version depends on libxml2, so soong/clangprebuilts.so
becomes slightly hackier.

Bug: 155835175
Test: OUT_DIR=out prebuilts/clang-tools/build-prebuilts.sh, then use generated prebuilts
Change-Id: I1daa4b4ab58ce504c958464aca169b14144cd22d
diff --git a/Android.bp b/Android.bp
index e339c7c..2d87828 100644
--- a/Android.bp
+++ b/Android.bp
@@ -35,3 +35,7 @@
 llvm_darwin_filegroup {
     name: "libc++abi_darwin",
 }
+
+llvm_darwin_filegroup {
+    name: "libxml2_darwin",
+}