[clang-tidy] New check: modernize-use-atomic-shared-ptr

C++20 gives us ability to use `std::atomic>`. Needs a check to convert legacy to use this ability.

Before:
```
std::shared_ptr sharedPtr;

void writer() {
auto newPtr = std::make_shared(42);
std::atomic_store(&sharedPtr, newPtr);
}

void reader() {
auto localPtr = std::atomic_load(&sharedPtr);
if (localPtr) {
std::cout << *localPtr << "\n"; // 42
}
}
```

After:
```
std::atomic> atomicPtr;

void writer() {
auto newPtr = std::make_shared(42);
atomicPtr.store(newPtr);
}

void reader() {
auto localPtr = atomicPtr.load();
if (localPtr) {
std::cout << *localPtr << "\n"; // 42
}
}
```

Start from the before-and-after C++20 examples in the issue and review the clang-tidy modernize-check infrastructure to determine the supported atomic_load and atomic_store forms. Done means the new check consistently converts eligible legacy shared_ptr operations to std::atomic> with equivalent load and store calls.

Written by the indexing model from the issue text.