Turn on color diagnostics for c++.

Change-Id: I4a0cde05d89399f39974760317c51021570b10e5
diff --git a/build/toolchain/host.gni b/build/toolchain/host.gni
index e2f8913..1ba74d7 100644
--- a/build/toolchain/host.gni
+++ b/build/toolchain/host.gni
@@ -136,7 +136,7 @@
   host_cc_toolchain("${target_name}_clang") {
     ar = "llvm-ar"
     cc = "clang -fcolor-diagnostics"
-    cxx = "clang++"
+    cxx = "clang++ -fcolor-diagnostics"
 
     # TODO: remove the need for this
     extra_defines = "-DPL011_BASE=0"