[analyzer][UninitializedObjectChecker] PR41590: Regard _Atomic types as primitive

[analyzer][UninitializedObjectChecker] PR41590: Regard _Atomic types as primitive

https://bugs.llvm.org/show_bug.cgi?id=41590

For the following code snippet, UninitializedObjectChecker crashed:

struct MyAtomicInt {
  _Atomic(int) x;
  MyAtomicInt() {}
};

void entry() {
  MyAtomicInt b;
}

The problem was that _Atomic types were not regular records, unions,
dereferencable or primitive, making the checker hit the llvm_unreachable at
lib/StaticAnalyzer/Checkers/UninitializedObject/UninitializedObjectChecker.cpp:347.
The solution is to regard these types as primitive as well. The test case shows
that with this addition, not only are we able to get rid of the crash, but we
can identify x as uninitialized.

Differential Revision: https://reviews.llvm.org/D61106

llvm-svn: 359230