Gate V3Covergroup pass on useCovergroup() flag

Add v3Global.useCovergroup() flag (following the useRandSequence()
pattern) that is set to true when a covergroup_declaration is parsed.
Gate the V3Covergroup::covergroup() pass in Verilator.cpp on this flag
so the pass is skipped entirely for designs with no covergroups.

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

src/V3Global.h

@@ -130,6 +130,7 @@ class V3Global final {
     bool m_hasSystemCSections = false;  // Has AstSystemCSection that need to be emitted
     bool m_useParallelBuild = false;  // Use parallel build for model
     bool m_useRandSequence = false;  // Has `randsequence`
+    bool m_useCovergroup = false;  // Has covergroup declarations
     bool m_useRandomizeMethods = false;  // Need to define randomize() class methods
     uint64_t m_currentHierBlockCost = 0;  // Total cost of this hier block, used for scheduling
 
@@ -213,6 +214,8 @@ public:
     void useParallelBuild(bool flag) { m_useParallelBuild = flag; }
     bool useRandSequence() const { return m_useRandSequence; }
     void useRandSequence(bool flag) { m_useRandSequence = flag; }
+    bool useCovergroup() const { return m_useCovergroup; }
+    void useCovergroup(bool flag) { m_useCovergroup = flag; }
     bool useRandomizeMethods() const { return m_useRandomizeMethods; }
     void useRandomizeMethods(bool flag) { m_useRandomizeMethods = flag; }
     void saveJsonPtrFieldName(const std::string& fieldName);

src/Verilator.cpp

@@ -226,7 +226,7 @@ static void process() {
 
         // Functional coverage code generation
         //    Generate code for covergroups/coverpoints
-        V3Covergroup::covergroup(v3Global.rootp());
+        if (v3Global.useCovergroup()) V3Covergroup::covergroup(v3Global.rootp());
 
         // Resolve randsequence if they are used by the design
         if (v3Global.useRandSequence()) V3RandSequence::randSequenceNetlist(v3Global.rootp());

src/verilog.y

@@ -6907,6 +6907,7 @@ covergroup_declaration<nodep>:  // ==IEEE: covergroup_declaration
         /*cont*/ yENDGROUP endLabelE
                         { AstClass *cgClassp = new AstClass{$<fl>2, *$2, PARSEP->libname()};
                           cgClassp->isCovergroup(true);
+                          v3Global.useCovergroup(true);
 
                           AstNode* sampleArgs = nullptr;