sv: support declaration in generate for initialization

This is accomplished by generating a unique name for the genvar,
renaming references to the genvar only in the loop's initialization,
guard, and incrementation, and finally adding a localparam inside the
loop body with the original name so that the genvar can be shadowed as
expected.

tests/verilog/genvar_loop_decl_1.sv

@@ -0,0 +1,18 @@
+`default_nettype none
+
+module gate(a);
+	for (genvar i = 0; i < 2; i++)
+		wire [i:0] x = '1;
+
+	output wire [32:0] a;
+	assign a = {1'b0, genblk1[0].x, 1'b0, genblk1[1].x, 1'b0};
+endmodule
+
+module gold(a);
+	genvar i;
+	for (i = 0; i < 2; i++)
+		wire [i:0] x = '1;
+
+	output wire [32:0] a;
+	assign a = {1'b0, genblk1[0].x, 1'b0, genblk1[1].x, 1'b0};
+endmodule