verilog: fix sizing of constant args for tasks/functions

- Simplify synthetic localparams for normal calls to update their width
    - This step was inadvertently removed alongside `added_mod_children`
- Support redeclaration of constant function arguments
    - `eval_const_function` never correctly handled this, but the issue
      was not exposed in the existing tests until the recent change to
      always attempt constant function evaluation when all-const args
      are used
- Check asserts in const_arg_loop and const_func tests
- Add coverage for width mismatch error cases

frontends/ast/ast.h

@@ -263,7 +263,13 @@ namespace AST
 		bool detect_latch(const std::string &var);
 
 		// additional functionality for evaluating constant functions
-		struct varinfo_t { RTLIL::Const val; int offset; bool is_signed; };
+		struct varinfo_t {
+			RTLIL::Const val;
+			int offset;
+			bool is_signed;
+			AstNode *arg = nullptr;
+			bool explicitly_sized;
+		};
 		bool has_const_only_constructs();
 		bool replace_variables(std::map<std::string, varinfo_t> &variables, AstNode *fcall, bool must_succeed);
 		AstNode *eval_const_function(AstNode *fcall, bool must_succeed);