klee/ADT: fixed c++20 warnings

Signed-off-by: Vitaly Chipounov <vitaly@chipounov.fr>

Author: vitalych

klee/include/klee/Internal/ADT/ImmutableSet.h

@@ -66,7 +66,7 @@ template <class T, class CMP = std::less<T>> class ImmutableSet {
     const value_type &max() const {
         return elts.max();
     }
-    size_t size() {
+    size_t size() const {
         return elts.size();
     }
 

klee/include/klee/Internal/ADT/ImmutableTree.h

@@ -119,7 +119,7 @@ template <typename T> class FixedStack {
     void pop_back() {
         --pos;
     }
-    bool empty() {
+    bool empty() const {
         return pos == 0;
     }
     T &back() {
@@ -133,10 +133,10 @@ template <typename T> class FixedStack {
         return *this;
     }
 
-    bool operator==(const FixedStack &b) {
+    bool operator==(const FixedStack &b) const {
         return (pos == b.pos && std::equal(elts, elts + pos, b.elts));
     }
-    bool operator!=(const FixedStack &b) {
+    bool operator!=(const FixedStack &b) const {
         return !(*this == b);
     }
 };
@@ -179,10 +179,10 @@ template <class K, class V, class KOV, class CMP> class ImmutableTree<K, V, KOV,
         return &n->value;
     }
 
-    bool operator==(const iterator &b) {
+    bool operator==(const iterator &b) const {
         return stack == b.stack;
     }
-    bool operator!=(const iterator &b) {
+    bool operator!=(const iterator &b) const {
         return stack != b.stack;
     }