Enable __is_trivially* intrinsics for GCC 5.1

Until GCC 5.1 the __is_trivially* intrinsics were not provided. Enable use of
the builtins for GCC 5.1.

Also enable Reference qualified member functions for GCC 4.9 and greater.

This patch also defines _GNUC_VER to 0 when __GNUC__ is not defined because
libc++ assumes _GNUC_VER is always defined.

llvm-svn: 239653