[ImportVerilog] Support handling the slang::ast::StatementBlockSymbol.

[hailongSun2000]

Now, we ignore statement block symbols. But in some cases, we'll trigger the dominance error. For example:

module top_module;
    always @(*) begin
        integer j;
        for(j = 0; j < 4; j++);
    end 
endmodule

Error:

forLoop.sv:2:12: error: operand #0 does not dominate this use
    always @(*) begin
           ^
forLoop.sv:2:12: note: see current operation: %20 = "moore.read"(%0) : (!moore.ref<l32>) -> !moore.l32
forLoop.sv:3:17: note: note: operand defined here (op in a parent region)
        integer j;

or

module top_module;
    always @(*) begin
        for(int i = 0;i<4;i++);
    end
endmodule

Error:

forLoop.sv:2:12: error: operand #0 does not dominate this use
    always @(*) begin
           ^
forLoop.sv:2:12: note: see current operation: %9 = "moore.read"(%1) : (!moore.ref<i32>) -> !moore.i32
forLoop.sv:3:17: note: operand defined here (op in a parent region)
        for(int i = 0;i<4;i++);

This error is caused by the moore.wait_event. When we handle always @(*), we must ensure which signal will lead to recalculating the whole always block. If we declare integer j out of the always block, we can generate the following Moore IR:

module {
  moore.module @top_module() {
    %j = moore.variable : <l32>
    moore.procedure always {
      moore.wait_event {
        %10 = moore.read %j : <l32>
        moore.detect_event any %10 : l32
      }
      %0 = moore.constant 0 : i32
      %1 = moore.conversion %0 : !moore.i32 -> !moore.l32
      moore.blocking_assign %j, %1 : l32
      cf.br ^bb1
    ^bb1:  // 2 preds: ^bb0, ^bb3
      %2 = moore.read %j : <l32>
      %3 = moore.constant 4 : i32
      %4 = moore.conversion %3 : !moore.i32 -> !moore.l32
      %5 = moore.slt %2, %4 : l32 -> l1
      %6 = moore.conversion %5 : !moore.l1 -> i1
      cf.cond_br %6, ^bb2, ^bb4
    ^bb2:  // pred: ^bb1
      cf.br ^bb3
    ^bb3:  // pred: ^bb2
      %7 = moore.read %j : <l32>
      %8 = moore.constant 1 : l32
      %9 = moore.add %7, %8 : l32
      moore.blocking_assign %j, %9 : l32
      cf.br ^bb1
    ^bb4:  // pred: ^bb1
      moore.return
    }
    moore.output
  }
}

The %10 = moore.read %j : <l32> means that we must have already created the %j = moore.variable : <l32>. So I think maybe we don't ignore slang::ast::StatementBlockSymbol.