Emit error when encountering assert/assume properties

Yosys doesn't have good handling of properties yet (see
YosysHQ/yosys#3223).  In the meantime, SymbiYosys with the UHDM plugin
will happily prove this model "correct":

    module wrong(input clk);
        logic data;
        always_comb data <= 0;
        assert property( @(posedge clk) data );
    endmodule

The `assert property` is silently dropped from the UHDM parse tree when
it's converted to AST nodes, and the user's design has a bug but passes
formal.  (Changing it to a clocked immediate assertion allows the solver
to find a counterexample.)

At least, emit an error for this case.  Real support for SVA assertions
can be added.

Signed-off-by: George Hilliard <thirtythreeforty@gmail.com>

uhdm-plugin/UhdmAst.cc

diff --git a/uhdm-plugin/UhdmAst.cc b/uhdm-plugin/UhdmAst.cc
index a08f513..ba4d8bc 100644
--- a/uhdm-plugin/UhdmAst.cc
+++ b/uhdm-plugin/UhdmAst.cc
@@ -1490,7 +1490,7 @@
                 }
             });
             visit_one_to_many({vpiModule, vpiInterface, vpiTaskFunc, vpiParameter, vpiParamAssign, vpiPort, vpiNet, vpiArrayNet, vpiGenScopeArray,
-                               vpiContAssign, vpiProcess, vpiClockingBlock},
+                               vpiContAssign, vpiProcess, vpiClockingBlock, vpiAssertion},
                               obj_h, [&](AST::AstNode *node) {
                                   if (node) {
                                       if (node->type == AST::AST_ASSIGN && node->children.size() < 2)
@@ -3897,6 +3897,10 @@
         if (!shared.no_assert)
             process_immediate_assert();
         break;
+    case vpiAssert:
+        if (!shared.no_assert)
+            process_unsupported_stmt(object);
+        break;
     case vpiHierPath:
         process_hier_path();
         break;
@@ -3935,6 +3939,9 @@
     case vpiImmediateAssume:
         process_immediate_assume();
         break;
+    case vpiAssume:
+        process_unsupported_stmt(object);
+        break;
     case vpiWhile:
         process_while();
         break;