c++: using in diagnostics [PR102987]

The decl pretty-printing code wasn't looking at the flags parameter, so we
were printing 'using' in the middle of an expression.

	PR c++/102987

gcc/cp/ChangeLog:

	* error.cc (dump_decl) [USING_DECL]: Respect flags.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/using1.C: Check pretty-printing.