Add SingleInlineArray read/_modify accessors

For mutable access.

Author: atrick

SwiftCompilerSources/Sources/SIL/Utilities/SequenceUtilities.swift

@@ -123,21 +123,35 @@ extension LazyFilterSequence : CollectionLikeSequence,
 //===----------------------------------------------------------------------===//
 
 public struct SingleInlineArray<Element>: RandomAccessCollection, FormattedLikeArray {
-  private var singleElement: Element? = nil
+  private var singleElement: Element?
   private var multipleElements: [Element] = []
 
   public init() {}
 
+  public init(element: Element) {
+    singleElement = element
+  }
+
   public var startIndex: Int { 0 }
   public var endIndex: Int {
     singleElement == nil ? 0 : multipleElements.count + 1
   }
 
   public subscript(_ index: Int) -> Element {
-    if index == 0 {
-      return singleElement!
+    _read {
+      if index == 0 {
+        yield singleElement!
+      } else {
+        yield multipleElements[index - 1]
+      }
+    }
+    _modify {
+      if index == 0 {
+        yield &singleElement!
+      } else {
+        yield &multipleElements[index - 1]
+      }
     }
-    return multipleElements[index - 1]
   }
 
   public mutating func push(_ element: Element) {