Put back LLVM_PREBUILTS_BASE.

Without this, WITH_TIDY is disabled because
PATH_TO_CLANG_TIDY uses LLVM_PREBUILTS_BASE.

Test: build with WITH_TIDY=1
Change-Id: I607ae45550f4574b3cee8e91c3d5b7eaa5f0a96c
diff --git a/core/clang/versions.mk b/core/clang/versions.mk
index d9c8aab..711e5a4 100644
--- a/core/clang/versions.mk
+++ b/core/clang/versions.mk
@@ -1,3 +1,4 @@
 ## Clang/LLVM release versions.
 
 LLVM_PREBUILTS_VERSION ?= clang-3289846
+LLVM_PREBUILTS_BASE ?= prebuilts/clang/host