[clang] improve diagnostics for misaligned and large atomics

[clang] improve diagnostics for misaligned and large atomics

"Listing the alignment and access size (== expected alignment) in the warning
seems like a good idea."

solves PR 46947

  struct Foo {
    struct Bar {
      void * a;
      void * b;
    };
    Bar bar;
  };

  struct ThirtyTwo {
    struct Large {
      void * a;
      void * b;
      void * c;
      void * d;
    };
    Large bar;
  };

  void braz(Foo *foo, ThirtyTwo *braz) {
    Foo::Bar bar;
    __atomic_load(&foo->bar, &bar, __ATOMIC_RELAXED);

    ThirtyTwo::Large foobar;
    __atomic_load(&braz->bar, &foobar, __ATOMIC_RELAXED);
  }

repro.cpp:21:3: warning: misaligned atomic operation may incur significant performance penalty; the expected (16 bytes) exceeds the actual alignment (8 bytes) [-Watomic-alignment]
  __atomic_load(&foo->bar, &bar, __ATOMIC_RELAXED);
  ^
repro.cpp:24:3: warning: misaligned atomic operation may incur significant performance penalty; the expected (32 bytes) exceeds the actual alignment (8 bytes) [-Watomic-alignment]
  __atomic_load(&braz->bar, &foobar, __ATOMIC_RELAXED);
  ^
repro.cpp:24:3: warning: large atomic operation may incur significant performance penalty; the access size (32 bytes) exceeds the max lock-free size (16  bytes) [-Watomic-alignment]
3 warnings generated.

Differential Revision: https://reviews.llvm.org/D85102