minor #19868 [Security] Optimize RoleHierarchy's buildRoleMap method (Enleur)v2.7.18

This PR was squashed before being merged into the 2.7 branch (closes #19868).

Discussion
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[Security] Optimize RoleHierarchy's buildRoleMap method

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Doc PR        | no

I have an issue with a large role hierarchy(~150 roles). Optimized it a little bit

![image](https://cloud.githubusercontent.com/assets/858989/18271257/df6c4ba0-7439-11e6-8406-e13bdcefe9ca.png)

Commits
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c3b68b0 [Security] Optimize RoleHierarchy's buildRoleMap method

1 files changed, 10 insertions, 2 deletions

diff --git a/Core/Role/RoleHierarchy.php b/Core/Role/RoleHierarchy.php
index 793007e..95e76ee 100644
--- a/Core/Role/RoleHierarchy.php
+++ b/Core/Role/RoleHierarchy.php
@@ -65,9 +65,17 @@ class RoleHierarchy implements RoleHierarchyInterface
                 }
 
                 $visited[] = $role;
-                $this->map[$main] = array_unique(array_merge($this->map[$main], $this->hierarchy[$role]));
-                $additionalRoles = array_merge($additionalRoles, array_diff($this->hierarchy[$role], $visited));
+
+                foreach ($this->hierarchy[$role] as $roleToAdd) {
+                    $this->map[$main][] = $roleToAdd;
+                }
+
+                foreach (array_diff($this->hierarchy[$role], $visited) as $additionalRole) {
+                    $additionalRoles[] = $additionalRole;
+                }
             }
+
+            $this->map[$main] = array_unique($this->map[$main]);
         }
     }
 }