c++: fix ICE on FUNCTION_DECLs inside coroutines [PR115906]

When register_local_var_uses iterates a BIND_EXPRs BIND_EXPR_VARS, it
fails to account for the fact that FUNCTION_DECLs might be present, and
later passes it to DECL_HAS_VALUE_EXPR_P.  This leads to a tree check
failure in DECL_HAS_VALUE_EXPR_P:

  tree check: expected var_decl or parm_decl or result_decl, have
  function_decl in register_local_var_uses

We only care about PARM_DECL and VAR_DECL, so select only those.

PR c++/115906 - [coroutines] missing diagnostic and ICE when co_await used as default argument in function declaration

gcc/cp/ChangeLog:

	PR c++/115906
	* coroutines.cc (register_local_var_uses): Only process
	PARM_DECL and VAR_DECLs.

gcc/testsuite/ChangeLog:

	PR c++/115906
	* g++.dg/coroutines/coro-function-decl.C: New test.