tests: add a test which checks that only a minimal ARC operations are generated

This is the case for the test since we enabled OSSA modules.
rdar://143691773

Author: eeckstein

test/SILOptimizer/optimal_arc.swift

@@ -0,0 +1,54 @@
+// RUN: %target-swift-frontend -module-name test -O -emit-sil -O -primary-file %s | %FileCheck %s
+
+// REQUIRES: optimized_stdlib,swift_stdlib_no_asserts
+
+// Check that there is only a single retain in concat.
+
+// CHECK-LABEL: sil hidden @$s4test6concatyAA14TestCollectionVAD_ADtF :
+// CHECK:         retain
+// CHECK-NOT:     retain
+// CHECK-NOT:     release
+// CHECK:         apply
+// CHECK-NOT:     retain
+// CHECK-NOT:     release
+// CHECK:       } // end sil function '$s4test6concatyAA14TestCollectionVAD_ADtF'
+func concat(_ l: TestCollection, _ r: TestCollection) -> TestCollection {
+    l + r
+}
+
+struct TestCollection: RandomAccessCollection, RangeReplaceableCollection {
+  private var base: [Int]
+
+  init(base: [Int]) {
+    self.base = base
+  }
+
+  init() {
+    self.base = []
+  }
+
+  var startIndex: Int { base.startIndex }
+
+  var endIndex: Int { base.endIndex }
+
+  subscript(index: Int) -> Int {
+    get {
+      base[index]
+    }
+    set {
+      base[index] = newValue
+    }
+  }
+
+  mutating func replaceSubrange<C>(_ range: Range<Int>, with newElements: C) where C: Collection, C.Element == Int {
+    self.base.replaceSubrange(range, with: newElements)
+  }
+
+  @inline(never)
+  mutating func append<S>(contentsOf other: S) where S: Sequence, S.Element == Int {
+    base.append(contentsOf: other)
+  }
+}
+
+
+