[clang-tidy] Raise minimum standard level for several checks from C++98 to C++11 (#152401)

`modernize-replace-auto-ptr`, `modernize-use-equals-delete`, and
`modernize-use-auto` use `std::unique_ptr`, `= delete`, and `auto`
respectively, which are all C++11 features.

The interesting bit is `modernize-use-nullptr`'s tests:

- Some relied on int-to-pointer conversions that were removed in C++11.
There are two variations here. First, tests like this, which become
ill-formed:
  ```cpp
  const int null = 0;
  void * ptr = null;
  ```
I just deleted these cases; if they're ill-formed, we're not losing
anything, right?
  Second, tests like this:
  ```cpp
  float * ptr = (float *)int(0.f);
  ```
These don't become ill-formed, but start generating a different AST. In
C++98, they are `NullToPointer` conversions, but in C++11, they become
generic `IntegralToPointer` conversions. I deleted these cases too,
though I'm less sure here.
- Folded `struct Bar` into `class A`, because those both suffer from the
same false negatives.

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