Fix up the BUILD_CLANG_ONLY code to work properly after r184794.

When BUILD_CLANG_ONLY is set to YES, it is supposed to simply limit the tools
that get built.  The change in r184794 broke this feature by moving libclang
and c-index-test into PARALLEL_DIRS. Those are both supposed to be in DIRS,
because c-index-test has a build dependency on libclang and cannot be
reliably built in parallel with it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187246 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/Makefile b/tools/Makefile
index 902e673..dddc07b 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -22,8 +22,9 @@
 OPTIONAL_PARALLEL_DIRS := extra
 
 ifeq ($(BUILD_CLANG_ONLY),YES)
-  PARALLEL_DIRS := driver libclang c-index-test
-  OPTIONAL_DIRS :=
+  DIRS := libclang c-index-test
+  PARALLEL_DIRS := driver
+  OPTIONAL_PARALLEL_DIRS :=
 endif
 
 include $(CLANG_LEVEL)/Makefile