Teach clang to look for libcxx in /usr/local/include/c++ on Linux

As The default CMAKE install prefix is /usr/local ( https://cmake.org/cmake/help/v3.0/variable/CMAKE_INSTALL_PREFIX.html ),
sudo ninja install ends up installing clang, LLVM and libcxx under /usr/local.
In development scenario, when clang is run from the build location it will not
find libcxx at neither (build location)/../include/c++ nor /usr/include/c++.
This patch lets development clang find system installed libcxx without adding
-isystem /usr/local/include/c++. Also addresses the FIXME by explaining the 
use-case for these include paths.

llvm-svn: 269801