Add -Wno-unused-command-line-argument to clang cflags to enable ccache.

Previously ccache is disabled when it fails calling clang's preprocessor with
unused arguments (such as '-Wa,--noexecstack') in the command line.

See http://petereisentraut.blogspot.com/2011/05/ccache-and-clang.html.
(-Qunused-arguments suppresses more than
-Wno-unused-command-line-argument does.)

Change-Id: I6cde307632c8395c053eb28063d7844d93070562

core/clang/config.mk

@@ -42,6 +42,11 @@ CLANG_CONFIG_EXTRA_CFLAGS += \
 CLANG_CONFIG_EXTRA_CFLAGS += \
   -Werror=int-conversion
 
+# Workaround for ccache with clang.
+# See http://petereisentraut.blogspot.com/2011/05/ccache-and-clang.html.
+CLANG_CONFIG_EXTRA_CFLAGS += \
+  -Wno-unused-command-line-argument
+
 CLANG_CONFIG_UNKNOWN_CFLAGS := \
   -finline-limit=64 \
   -fno-canonical-system-headers \