[clang] [analyzer] Use AllocaRegion in MallocChecker (PR #72402)

[Artem Dergachev via cfe-commits]

================
@@ -266,13 +266,18 @@ void CheckUseZeroAllocated1(void) {
 }
 
 char CheckUseZeroAllocated2(void) {
+  // FIXME: The return value of `alloca()` is modeled with `AllocaRegion`
+  // instead of `SymbolicRegion`, so the current implementation of
+  // `MallocChecker::checkUseZeroAllocated()` cannot handle it; and we get an
+  // unrelated, but suitable warning from core.uninitialized.UndefReturn.
   char *p = alloca(0);
-  return *p; // expected-warning {{Use of memory allocated with size zero}}
+  return *p; // expected-warning {{Undefined or garbage value returned to caller}}
----------------
haoNoQ wrote:

So we'd have no warning in case of
```c++
char CheckUseZeroAllocatedAndInitialized(void) {
   char *p = alloca(0);
  *p = 4;
   return *p;
}
```
? Might be worth testing.

(It's probably not hard to fix it as well? It's not like `AllocaRegion` is special when it comes to being able to carry dynamic extent?)

https://github.com/llvm/llvm-project/pull/72402