c++: OVERLOAD in diagnostics

In modules we can get an OVERLOAD around a non-function, so let's tail
recurse instead of falling through.  As a result we start printing the
template header in this testcase.

gcc/cp/ChangeLog:

	* error.cc (dump_decl) [OVERLOAD]: Recurse on single case.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/pr61945.C: Adjust diagnostic.