clang++ flags: not used but to keep TARGET_GLOBAL_CPPFLAGS from clang++
so far... but they are there to use, with this commit:
1) Added TARGET_GLOBAL_CLANGPP_FLAGS & filter-out CLANG_CONFIG_UNKNOWN_CFLAGS
from TARGET_GLOBAL_CPPFLAGS. NOTE: Prior to this, if an unknown clang flag
was in TARGET_GLOBAL_CPPFLAGS you would get a clang++ error.

2) Added TARGET_GLOBAL_CLANGPP_FLAGS to clang-flag-subst

3) Added CLANG_CONFIG_EXTRA_CPPFLAGS += LOCAL_CPPFLAGS in LOCAL_CLANG use.

4) 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 & LDFLAGS

Change-Id: I3ed147f180267db975ee556a4730c2c1dad2d554