Safer replacement of non-synth constructs

[alaindargelas]

Simply removing a non-synthesizable construct from the AST can lead to ill-formed parent statements:
Example:
for (int i = 0; .....)
$fdisplay (...) ;

Would become:

for (int i = 0; .....)

And crash during Synthesis due to incomplete for loop statement (No actual statement).

Since the null_stmt is not supported in downstream tools, the safest statement is a dummy $display statement to put in place of the non-supported statement.