c++: is_this_parameter and coroutines proxies

Compiling coroutines/pr95736.C with the implicit constexpr patch broke
because is_this_parameter didn't recognize the coroutines proxy for 'this'.

gcc/cp/ChangeLog:

	* semantics.c (is_this_parameter): Check DECL_HAS_VALUE_EXPR_P
	instead of is_capture_proxy.