envsetup.sh: Export LLVM_AOSP_PREBUILTS_VERSION with the current clang version from soong

Needed for KERNEL_CLANG_VERSION if TARGET_KERNEL_CLANG_VERSION hasn't been set by the device config

It's boring to change it on each Google change ...

Change-Id: I1d682611cbf6c60d7ae6227b0b5afde591adf471
diff --git a/build/envsetup.sh b/build/envsetup.sh
index db91338..7ff4c60 100644
--- a/build/envsetup.sh
+++ b/build/envsetup.sh
@@ -12,6 +12,9 @@
 EOF
 }
 
+CLANG_VERSION=$(build/soong/scripts/get_clang_version.py)
+export LLVM_AOSP_PREBUILTS_VERSION="${CLANG_VERSION}"
+
 # check to see if the supplied product is one we can build
 function check_product()
 {