Fix infinite recursion in visit(AstCovergroup*) and fileline

Two bugs in the covergroup -> AstClass transformation in V3LinkParse:

1. Infinite recursion: when a covergroup has a clocking event (e.g.
   `@(posedge clk)`), visit(AstCovergroup*) embeds a sentinel
   AstCovergroup node inside the new AstClass to carry the event for
   V3Covergroup.cpp. The subsequent iterate(cgClassp) call then visits
   the sentinel via visit(AstNodeModule*) -> iterateChildren -> which
   hits visit(AstCovergroup*) again, creating another class with another
   sentinel, infinitely.

   Fix: skip transformation in visit(AstCovergroup*) when already inside
   a covergroup class (m_modp->isCovergroup()), so sentinel nodes are
   left alone.

2. Wrong fileline column: AstCovergroup was created with the fileline of
   the identifier token ($<fl>2, the name position) rather than the
   'covergroup' keyword token ($<fl>1). This caused warnings about the
   covergroup to point to the name column instead of the keyword column.

   Fix: use $<fl>1 (the 'covergroup' keyword fileline) when constructing
   AstCovergroup in the parser.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/V3LinkParse.cpp

@@ -1151,6 +1151,10 @@ class LinkParseVisitor final : public VNVisitor {
     }
 
     void visit(AstCovergroup* nodep) override {
+        // If we're already inside a covergroup class, this is the sentinel AstCovergroup
+        // node carrying the clocking event for V3Covergroup — don't re-transform it.
+        if (m_modp && VN_IS(m_modp, Class) && VN_CAST(m_modp, Class)->isCovergroup()) return;
+
         // Transform raw parse-time AstCovergroup into a fully-formed AstClass
         cleanFileline(nodep);
 

src/verilog.y

@@ -6913,7 +6913,7 @@ covergroup_declaration<nodep>:  // ==IEEE: covergroup_declaration
                               else
                                   sampleArgsp = $4;
                           }
-                          $$ = new AstCovergroup{$<fl>2, *$2, static_cast<AstVar*>($3),
+                          $$ = new AstCovergroup{$<fl>1, *$2, static_cast<AstVar*>($3),
                                                  static_cast<AstVar*>(sampleArgsp), $6, clockp};
                           GRAMMARP->endLabel($<fl>8, $$, $8); }
         |        yCOVERGROUP yEXTENDS idAny ';'