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
1 file changed