Internals: Rename Ast on non-node classes (#3262). No functional change.

This commit has the following replacements applied:

	s/\bAstUserInUseBase\b/VNUserInUseBase/g;
        s/\bAstAttrType\b/VAttrType/g;
        s/\bAstBasicDTypeKwd\b/VBasicDTypeKwd/g;
        s/\bAstDisplayType\b/VDisplayType/g;
        s/\bAstNDeleter\b/VNDeleter/g;
        s/\bAstNRelinker\b/VNRelinker/g;
        s/\bAstNVisitor\b/VNVisitor/g;
        s/\bAstPragmaType\b/VPragmaType/g;
        s/\bAstType\b/VNType/g;
        s/\bAstUser1InUse\b/VNUser1InUse/g;
        s/\bAstUser2InUse\b/VNUser2InUse/g;
        s/\bAstUser3InUse\b/VNUser3InUse/g;
        s/\bAstUser4InUse\b/VNUser4InUse/g;
        s/\bAstUser5InUse\b/VNUser5InUse/g;
        s/\bAstVarType\b/VVarType/g;

src/V3MergeCond.cpp

@@ -60,7 +60,7 @@ enum class Mergeable {
     NO_IMPURE  // Tree cannot be merged because it contains an impure node
 };
 
-class CheckMergeableVisitor final : public AstNVisitor {
+class CheckMergeableVisitor final : public VNVisitor {
 private:
     // STATE
     bool m_condAssign = false;  // Does this tree contain an assignment to a condition variable??
@@ -109,7 +109,7 @@ public:
     }
 };
 
-class MarkVarsVisitor final : public AstNVisitor {
+class MarkVarsVisitor final : public VNVisitor {
 private:
     // METHODS
     VL_DEBUG_FUNC;  // Declare debug()
@@ -126,13 +126,13 @@ public:
     void mark(AstNode* node) { iterate(node); }
 };
 
-class MergeCondVisitor final : public AstNVisitor {
+class MergeCondVisitor final : public VNVisitor {
 private:
     // NODE STATE
     // AstVar::user1        -> Flag set for variables referenced by m_mgCondp
     // AstNode::user2       -> Flag marking node as included in merge because cheap to duplicate
-    const AstUser1InUse m_user1InUse;
-    const AstUser2InUse m_user2InUse;
+    const VNUser1InUse m_user1InUse;
+    const VNUser2InUse m_user2InUse;
 
     // STATE
     VDouble0 m_statMerges;  // Statistic tracking