Workaround for gcc-11.4/draft 2x spec flaw (#4679)

- A late draft of the C++2x spec introduced a non-backwards compatible
  change in the way overloading for operator ==/!= is handled; this was
  considered a defect and was fix in the final C++20 spec.

ir/id.h

@@ -47,6 +47,10 @@ struct ID : Util::IHasSourceInfo {
     }
     bool operator==(const ID &a) const { return name == a.name; }
     bool operator!=(const ID &a) const { return name != a.name; }
+    bool operator==(cstring a) const { return name == a; }
+    bool operator!=(cstring a) const { return name != a; }
+    bool operator==(const char *a) const { return name == a; }
+    bool operator!=(const char *a) const { return name != a; }
     explicit operator bool() const { return name; }
     operator cstring() const { return name; }
     bool isDontCare() const { return name == "_"; }

ir/namemap.h

@@ -130,7 +130,7 @@ class NameMap : public Node {
     }
 
     IRNODE_SUBCLASS(NameMap)
-    bool operator==(const Node &a) const override { return a == *this; }
+    bool operator==(const Node &a) const override { return a.operator==(*this); }
     bool operator==(const NameMap &a) const { return symbols == a.symbols; }
     bool equiv(const Node &a_) const override {
         if (static_cast<const Node *>(this) == &a_) return true;

lib/cstring.h

@@ -197,6 +197,9 @@ class cstring {
     bool operator==(cstring a) const { return str == a.str; }
     bool operator!=(cstring a) const { return str != a.str; }
 
+    bool operator==(std::nullptr_t) const { return str == nullptr; }
+    bool operator!=(std::nullptr_t) const { return str != nullptr; }
+
     // Other comparisons and tests. Linear time.
     bool operator==(const char *a) const { return str ? a && !strcmp(str, a) : !a; }
     bool operator!=(const char *a) const { return str ? !a || !!strcmp(str, a) : !!a; }