Filter out tools and unittests which we don't want to build when we're building clang only.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164916 91177308-0d34-0410-b5e6-96231b3b80d8

Makefile

@@ -27,6 +27,11 @@ ifeq ($(MAKECMDGOALS),libs-only)
   DIRS := $(filter-out tools docs, $(DIRS))
   OPTIONAL_DIRS :=
 endif
+ifeq ($(MAKECMDGOALS),clang-only)
+  DIRS := $(filter-out tools docs unittests, $(DIRS))
+	tools/driver tools/libclang
+  OPTIONAL_DIRS :=
+endif
 
 ###
 # Common Makefile code, shared by all Clang Makefiles.