verific: Fix enum_values support and signed attribute values

This uses the same constant parsing for enum_values and for attributes
and extends it to handle signed values as those are used for enums that
implicitly use the int type.

tests/verific/enum_values.sv

@@ -0,0 +1,37 @@
+typedef enum {
+    WA, WB, WC, WD
+} wide_state_t;
+
+typedef enum logic [1:0] {
+    A = 3, B = 0, C, D
+} state_t;
+
+module top(input clk, output z);
+
+    wide_state_t wide_state = WA;
+
+    always @(posedge clk) begin
+        case (wide_state)
+            WA: wide_state <= WB;
+            WB: wide_state <= WC;
+            WC: wide_state <= WD;
+            default: wide_state <= WA;
+        endcase
+    end
+
+    (* some_attribute = shortint'(42) *)
+    (* another_attribute = -1 *)
+    state_t state = A;
+
+    always @(posedge clk) begin
+        case (state)
+            A: state <= B;
+            B: state <= C;
+            C: state <= D;
+            default: state <= A;
+        endcase
+    end
+
+    assign z = (wide_state == WB) ^ (state == B);
+
+endmodule