Add TARGET_GLOBAL_CLANGPP_FLAGS ONLY to do:
1) filter-out CLANG_CONFIG_UNKNOWN_CFLAGS from TARGET_GLOBAL_CPPFLAGS for clang++
Prior to this, if an unknown clang flag was in TARGET_GLOBAL_CPPFLAGS you
would get a clang++ error.

2) Also Added TARGET_GLOBAL_CLANGPP_FLAGS to clang-flag-subst

3) Reduce space wasted by indirect duplicate rules:
ifneq ($(TARGET_CLANG_VERSION),)
ifeq ($(LOCAL_CLANG),true)

&&

ifeq ($(TARGET_CLANG_VERSION),)
ifeq ($(LOCAL_CLANG),true)

Change-Id: I38ae521882537df4cc4627fe1ca7145e7dc9a31b
My goto-dir ONLY FLAGS

Change-Id: I3ed147f180267db975ee556a4730c2c1dad2d554