[OpenACC] Improve C++20 compatibility of private/firstprivate test (#152233)

Under C++17, `DeletedCopy` and `DefaultedCopy` in the test
`clang/test/SemaOpenACC/private_firstprivate_reduction_required_ops.cpp`
are eligible for aggregate initialization. Under C++20 and up, that is
no longer the case. Therefore, an explicit constructor must be declared
to avoid additional diagnostics which the test does not expect.

This test was failing in our downstream testing where our toolchain uses
C++20 as the default dialect.

See this reduced example for more details:
https://godbolt.org/z/46oErYT43

Author: dgg5503

clang/test/SemaOpenACC/private_firstprivate_reduction_required_ops.cpp

@@ -37,10 +37,12 @@ struct ImplicitDelDtor {
 };
 
 struct DeletedCopy {
+  DeletedCopy();
   DeletedCopy(const DeletedCopy&) = delete;
 };
 
 struct DefaultedCopy {
+  DefaultedCopy();
   DefaultedCopy(const DefaultedCopy&) = default;
 };
 struct UserCopy {