Make tcg_dus behave more sanely; fixes a mkUsageInfo panic

The tcg_dus field used to contain *uses* of type and class decls,
but not *defs*.  That was inconsistent, and it really went wrong
for Template Haskell bracket.  What happened was that
 foo = [d| data A = A
       	   f :: A -> A
       	   f x = x |]
would find a "use" of A when processing the top level of the module,
which in turn led to a mkUsageInfo panic in MkIface.  The cause was
the fact that the tcg_dus for the nested quote didn't have defs for
A.