Add SingleInlineArray utility.

SwiftCompilerSources/Sources/Basic/Utils.swift

@@ -114,6 +114,37 @@ public struct StringRef : CustomStringConvertible, NoReflectionChildren {
   public static func ~=(pattern: StaticString, value: StringRef) -> Bool { value == pattern }
 }
 
+//===----------------------------------------------------------------------===//
+//                      Single-Element Inline Array
+//===----------------------------------------------------------------------===//
+
+public struct SingleInlineArray<Element>: RandomAccessCollection {
+  private var singleElement: Element? = nil
+  private var multipleElements: [Element] = []
+
+  public init() {}
+
+  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!
+    }
+    return multipleElements[index - 1]
+  }
+
+  public mutating func push(_ element: Element) {
+    guard singleElement != nil else {
+      singleElement = element
+      return
+    }
+    multipleElements.append(element)
+  }
+}
+
 //===----------------------------------------------------------------------===//
 //                            Bridging Utilities
 //===----------------------------------------------------------------------===//