[silgen] In SILGenApply.cpp, consistently indent classes in LLVM style.

Some of the classes in this file were using LLVM style for long classes, i.e.:

```
namespace {
class Foo {
  ... long class ...
};
} // end anonymous namespace
```

Other places, we were adding a level of indentation, i.e.:

```
namespace {
  class Foo {
    ... long class ...
  };
} // end anonymous namespace
```

This PR just standardizes the classes in this file that follow the later style,
to instead follow the former style.