gen_stub: stop cloning ExposedDocComment objects

The objects are immutable, and thus can be safely reused when held by an object
that gets cloned. Remove the unneeded cloning, and document this fact about the
class, noting that the property should be considered `readonly`.

Not actually using `readonly` to maintain PHP 7.4 support.

Author: DanielEScherzer

build/gen_stub.php

@@ -2099,9 +2099,6 @@ public function __clone()
         foreach ($this->framelessFunctionInfos as $key => $framelessFunctionInfo) {
             $this->framelessFunctionInfos[$key] = clone $framelessFunctionInfo;
         }
-        if ($this->exposedDocComment) {
-            $this->exposedDocComment = clone $this->exposedDocComment;
-        }
     }
 }
 
@@ -3146,9 +3143,6 @@ public function __clone()
         foreach ($this->attributes as $key => $attribute) {
             $this->attributes[$key] = clone $attribute;
         }
-        if ($this->exposedDocComment) {
-            $this->exposedDocComment = clone $this->exposedDocComment;
-        }
     }
 }
 
@@ -4011,10 +4005,6 @@ public function __clone()
         foreach ($this->attributes as $key => $attribute) {
             $this->attributes[$key] = clone $attribute;
         }
-
-        if ($this->exposedDocComment) {
-            $this->exposedDocComment = clone $this->exposedDocComment;
-        }
     }
 
     /**
@@ -4190,8 +4180,10 @@ public function getVariableName(): string {
     }
 }
 
+// Instances of ExposedDocComment are immutable and do not need to be cloned
+// when held by an object that is cloned
 class ExposedDocComment {
-    private string $docComment;
+    private /* readonly */ string $docComment;
 
     public function __construct(string $docComment) {
         $this->docComment = $docComment;