[clang] [analyzer] Conversion to CheckerFamily: NullabilityChecker (PR #143735)

[Donát Nagy via cfe-commits]

================
@@ -710,29 +706,28 @@ void NullabilityChecker::checkPreStmt(const ReturnStmt *S,
   Nullability RetExprTypeLevelNullability =
         getNullabilityAnnotation(lookThroughImplicitCasts(RetExpr)->getType());
 
-  bool NullReturnedFromNonNull = (RequiredNullability == Nullability::Nonnull &&
----------------
NagyDonat wrote:

I refactored this code block because the name of this local boolean `NullReturnedFromNonNull` was too similar to the checker frontend `NullReturnedFromNonnull` (which I introduced).

I eliminated the boolean instead of renaming it, because I feel that the old
```c++
bool B = some condition
if (B && other conditions) {
  ...
}
if (B) {
...
}
```
was less readable than the structure
```c++
if (some conditions) {
  if (other conditions) {
    ...
  }
  ...
}
```
that I'm introducing. (Deeply nested blocks are not ideal, but breaking up a logically coherent block into multiple `if` statements is even worse.)

https://github.com/llvm/llvm-project/pull/143735