Reposition clang-builtin include/ to immediately precede /usr/include/,

matching gcc compiler. Fixes #include_next <...> shenanigans that lead to
file-not-found failures with <cstddef> on libstdc++ 4.3.[012].

Updated C++ include header search paths for various Debian/Ubuntu and Fedora
linux distros.

llvm-svn: 103177