Update CHANGELOG.md for Swift 5.5 release (#39379)

<https://swift.org/blog/swift-5-5-released/>

Author: benrimmington

CHANGELOG.md

@@ -5,6 +5,7 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 
 Swift 5.6
 ---------
+
 * [SE-0315][]:
 
   Type expressions and annotations can now include "type placeholders" which
@@ -34,6 +35,8 @@ Swift 5.6
 Swift 5.5
 ---------
 
+### 2021-09-20 (Xcode 13.0)
+
 * [SE-0313][]:
 
   Parameters of actor type can be declared as `isolated`, which means that they