[interop][SwiftToCxx] NFC, add test coverage for resilient mutating method

Author: hyp

test/Interop/SwiftToCxx/structs/resilient-struct-in-cxx-execution.cpp

@@ -40,6 +40,11 @@ int main() {
 // CHECK: find - small dump
 // CURRENT-NEXT: x = 66
 // CHANGE-NEXT: x&y = 0&65
+  copySmallStruct.mutate();
+  copySmallStruct.dump();
+// CHECK: find - small dump
+// CURRENT-NEXT: x = 132
+// CHANGE-NEXT: x&y = 0&4294967230
 
   printSmallAndLarge(smallStruct, largeStruct);
 // CHECK: find - small dump

test/Interop/SwiftToCxx/structs/resilient-struct-in-cxx.swift

@@ -16,6 +16,13 @@ public struct FirstSmallStruct {
         print("x&y = \(x)&\(y)")
 #else
         print("x = \(x)")
+#endif
+    }
+
+    public mutating func mutate() {
+        x = x * 2
+#if CHANGE_LAYOUT
+        y = ~y
 #endif
     }
 }
@@ -61,6 +68,9 @@ public struct FirstSmallStruct {
 // CHECK-NEXT: inline void FirstSmallStruct::dump() const {
 // CHECK-NEXT:   return _impl::$s7Structs16FirstSmallStructV4dumpyyF(_getOpaquePointer());
 // CHECK-NEXT: }
+// CHECK-NEXT: inline void FirstSmallStruct::mutate() {
+// CHECK-NEXT:   return _impl::$s7Structs16FirstSmallStructV6mutateyyF(_getOpaquePointer());
+// CHECK-NEXT: }
 
 @frozen public struct FrozenStruct {
     private let storedInt: Int32