Allow const nodes in VNRef

No functional change.

src/V3Ast.h

@@ -2150,7 +2150,7 @@ static_assert(sizeof(VNRef<AstNode>) == sizeof(std::reference_wrapper<AstNode>),
 // without having to copy nodes into the collections.
 
 // Forward declaration to avoid including V3Hasher.h which needs V3Ast.h (this file).
-size_t V3HasherUncachedHash(AstNode&);
+size_t V3HasherUncachedHash(const AstNode&);
 
 // Specialization of std::hash for VNRef
 template <typename T_Node>  //

src/V3Hasher.cpp

@@ -530,6 +530,6 @@ V3Hash V3Hasher::uncachedHash(const AstNode* nodep) {
 // This is used by the std::hash specialization for VNRef.
 // Declared separately to avoid a circular header dependency.
 
-size_t V3HasherUncachedHash(AstNode& node) {
+size_t V3HasherUncachedHash(const AstNode& node) {
     return static_cast<size_t>(V3Hasher::uncachedHash(&node).value());
 }