more macOS Clang assume warnings

[Mattias Engdegård]

The latest assume reformulation caused several warnings like the following when 
building with Clang:

../../emacs/src/xwidget.h:171:72: warning: control reaches end of non-void
      function [-Wreturn-type]
INLINE struct xwidget *lookup_xwidget (Lisp_Object obj) { eassume (0); }
                                                                       ^
../../emacs/src/eval.c:1571:1: warning: function declared 'noreturn' should not
      return [-Winvalid-noreturn]
}
^

This is with Apple clang version 11.0.0 (clang-1100.0.33.17) but it looks like 
the same applies to clang trunk.