Rename libc++ internal templates __is_void, __is_integral,...

Rename libc++ internal templates __is_void, __is_integral, __is_floating_point, __is_pointer, __is_function_imp, __is_function, __is_member_function_pointer, __is_member_pointer, __is_signed_impl, __is_signed, __is_unsigned_impl, __is_unsigned to __libcpp_is_XXX, because some compilers have decided that __is_XXX are keywords. No functionality change.

llvm-svn: 199184