fmt: FmtPart::{STRING→LITERAL},{CHARACTER→STRING}.

Before this commit, the `STRING` variant inserted a literal string;
the `CHARACTER` variant inserted a string. This commit renames them
to `LITERAL` and `STRING` respectively.

frontends/ast/genrtlil.cc

@@ -790,7 +790,7 @@ struct AST_INTERNAL::ProcessGenerator
 				Fmt fmt;
 				fmt.parse_verilog(args, /*sformat_like=*/false, default_base, /*task_name=*/ast->str, current_module->name);
 				if (ast->str.substr(0, 8) == "$display")
-					fmt.append_string("\n");
+					fmt.append_literal("\n");
 				fmt.emit_rtlil(cell);
 			} else if (!ast->str.empty()) {
 				log_file_error(ast->filename, ast->location.first_line, "Found unsupported invocation of system task `%s'!\n", ast->str.c_str());

frontends/ast/simplify.cc

@@ -1079,7 +1079,7 @@ bool AstNode::simplify(bool const_fold, int stage, int width_hint, bool sign_hin
 				// when $display()/$write() functions are used in an initial block, print them during synthesis
 				Fmt fmt = processFormat(stage, /*sformat_like=*/false, default_base, /*first_arg_at=*/0, /*may_fail=*/true);
 				if (str.substr(0, 8) == "$display")
-					fmt.append_string("\n");
+					fmt.append_literal("\n");
 				log("%s", fmt.render().c_str());
 			}