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
1 file changed