Add test case for coro.alloca.free fix

Author: aschwaighofer

test/IRGen/modifyaccessor.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -emit-ir -disable-llvm-optzns -primary-file %s | %FileCheck %s
+extension Dictionary {
+  subscript(alternate key: Key) -> Value? {
+    get {
+      return self[key]
+    }
+    _modify {
+      var value: Value? = nil
+      yield &value
+    }
+  }
+}
+
+// CHECK-LABEL: define {{.*}}SSD14modifyaccessorE9alternateq_Sgx_tciM
+// CHECK: [[COROALLOCA:%.*]] = call token @llvm.coro.alloca.alloc
+// CHECK: call void @llvm.coro.alloca.free(token [[COROALLOCA]])