Use correct clang project location.

This project has been moved under separete project tree in external/clang
diff --git a/llvm.mk b/llvm.mk
index b181071..233223f 100644
--- a/llvm.mk
+++ b/llvm.mk
@@ -10,6 +10,6 @@
 LLVM_GEN_INTRINSICS_MK := $(LLVM_ROOT_PATH)/llvm-gen-intrinsics.mk
 LLVM_TBLGEN_RULES_MK := $(LLVM_ROOT_PATH)/llvm-tblgen-rules.mk
 
-CLANG_ROOT_PATH := $(LLVM_ROOT_PATH)/tools/clang
+CLANG_ROOT_PATH := external/clang
 
 include $(CLANG_ROOT_PATH)/clang.mk