c-decl.c rewrite (2/3) no more different_binding_level/different_tu

[Zack Weinberg]

1) This is the sort of code that no one ever writes except as part of
   a conformance suite.

2) This was going to break anyway with patch #3.  The bugs that patch
   #3 fixes are much more important.

3) Having multiple DECLs for the same object is untenable; as I said,
   it breaks too many assumptions in the language-independent parts of
   the compiler.

Therefore, I solicit ideas how to address this WITHOUT putting it back
the way it was; but I'm going to press on.  You might want to wait
till after patch #3, which is going to change the surrounding
structure quite a bit.

Please also file a PR.

zw